Dudley vantage flush valve problem

Concealed cistern, this one has pan fan and is operated by IR button if that makes any difference. The usual problem of letting by into pan, we are in a hard water area and things do get scaled up.
I have replaced the niagara dual pneumatic flush valve yet still on occasion the valve will stick up in the air- repeat flushing will eventually result in a clunk as it drops back into place.
I have tried it with an ordinary button and the same thing happens, so I think this rules out the non-return valves in button.
Any ideas what it could be?

 

Personally I end up taking the whole flush valve assembly out and descaling it. Descaler in hot or warm water and leave it for 15 mins. Clean it all and put it back again. Normally works.

 

Have replaced the flush valve ..
This is one with the cradle built in and it’s a reduced depth unit so would need to remove cistern ( panels off pan out yada yada) to attend to the valve seat.
Thinking maybe put tennis ball in to try and holds bit of descaler in cistern, but would have to tie it to string... can see it not working and/or going horribly wrong..

 

TBH it more sounds like it's not seated properly or its out of square if its holding up. IS there anything else touching it?

 

It’s not a seating issue. Just occasionally the valve doesn’t drop back down. It’s a crappy design with parts too close together. I was wondering if silicone lubricant or something similar might help? Any thoughts on that?

 

As its in water, you can only grease it. A hi tack silicone grease might work but I'd put it on and then wipe it off. WHat will be left will do a job.