LOFT CONVERSION done BUT 2 out of 3 loft rads not heating

I've had the installer back for this one already so want to try to get as many facts from the experts as to what might be going on.
New loft conversion, Vaillant EcoTec 37KW boiler on ground floor with UFH in kitchen and 13 rads + the 3 in the loft.
Rad feeds for loft have been taken off the rad circuit on first floor and go up into loft. According to the installer, due to the steelwork in the loft the plastic plumbing pipe runs had to go 'over' one of the steels (as it couldn't go through it of course) then back down into the loft floor void. I estimate the the feed pipes had to go over a 20cm high steel if that helps.
After installation and testing all 3 rads were working however after the heating was switched off (not needed due to time of year) 2 of the loft rads stopped heating. They are both downstream of the first one on the circuit that has always worked.
The rads (one a conventional convector rad the other a towel rail) were stone cold as were the rad pipe feeds to them. Bled both rads but NO air coming was out of their bleed valves, only water.
Plumber returned and said this was caused by the bend in the plastic feed pipe producing an air lock as it was going over the steel. He came and sorted that (didn't see what he did but he did say there was air trapped in the system) and then all 3 were working again.
A few days ago the 2 downstream stopped heating again, and as before with no air out of any of their rad bleed valves, only water.
Interestingly this time though the pipe feed to the TRV of the rad immediately downstream of the one that has always heated was hot hot but the one from the lock shield was cold. So, it seems as if some hot water has managed to get from the first rad on the circuit to this one, ie. better than last time it failed. (Note that before when the 2 rads failed their pipe feeds [TRV&Lockshield feeds] were both stone cold).
So, anyone got any ideas what's going on here.
Is it a fault in the pipe run design with the pipe going over the steel? If so can this air lock be sorted by way of an air valve on this pipe? If so what are the implications on the pressure in the boiler system overall if air is continually venting out of the sealed system?
OR might it be that the installer simply hasn't bothered to balance the system?
My concern is that seasonally as the central heating is switched off we will get this air lock (if indeed that's what it is) every year and will need a plumber out each time to sort it which isn't acceptable.
I've yet to pay the final loft build instalment so with some help from this forum as to what might be going on I will have stronger grounds for withholding final payment until this is well and truly sorted.
Thanks, over to the experts.....

It could well be the pipe running over the steel work is an air trap. Though of course it may not be air it could be hydrogen if the system if the system is in a state of decay. Make sure you have inhibitor in the system to stop this. Whoever installed the system should have treated it and should be able to tell you what inhibitor they have used. Not all inhibitors can be mixed together so don't just put anything in, it may act against what already in there.
Cutting an automatic air vent into the pipe that is becoming air locked should stop that from occurring. This should be neither difficult or expensive and I'm sure whoever installed the system would be more than willing to do this if you are withholding a completion payment.

Don't suppose you have any pics of the offending pipes ?

Could also be air ingress in addition to having a high point where air is being trapped. Are there automatic vents elsewhere on the system?

You could fit an auto air vent on the inverted U, but, quite honestly, I'd rather have a manual one (less to go wrong) as once the air is out of the system, it normally stays out, especially on a sealed system so you'll probably never need to use it again. Especially if it isn't an 'open-vented system'.

On a newly filled system there are often airlocks and air tends to find its way to the highest point and settle out. In your case, it probably causes a blockage in the process. You'll probably find it will be absolutely fine once the airlock has been cleared a couple of times though. If this weren't a new install, I wouldn't be insisting on changing it, but, as it is, manual air vent would be good practice. Or auto airvent.

Agree with Shaun though: pictures would be good. Otherwise we can only be vague as we can't see the situation.

Thanks all. Looking at the photo (as opposed to what the installer told me about the pipes going over the steels) it's clear that they come up from the first floor then via a 90 degree bend go under the loft floorboards and not over the steels. So, can air build up with this type of pipe installation? As for auto air vents I don't think we have one. Having said that B.Gas installed a valve on a heating pipe near boiler some years back as the Vaillant was losing pressure. This device has a gauge which was set on it and serves to simply maintain boiler pressure which we were losing at that time causing boiler cut outs (it turned out the pressure loss was due to a leak in a CH pipe, now repaired). However, I don't think this is a vent valve as Marnix & Ric2013 mentioned. The pipes shown in photo are now inaccessible behind a dwarf wall in loft but could an air vent be placed in the pipe run from first floor to loft (which is accessible in a cupboard below) or must it be in the highest part of the system? Where or what is the inverted U Ric2013?
As for inhibitors, yes he put it in but he never checked or asked for compatibility so not sure if that's the issue here?
Thanks for help so far, any further advice about balancing maybe??

Have they balanced the system after the additions?
Have you tried turning all the unaffected rads off and monitoring the result on the offending rads.

As above really, did you try balancing the system? Basically turn the rads off which are working to force all the heat into the ones which don't work

I'm pretty sure the installer hasn't done that so thanks for that advice, that's where I think I'll start before considering cutting in air valves etc. Thanks Matchless.plumb