I'm going to agree with the air camp. On the grounds that, while water is lazy and finds the shortest route, my limited experience with flow and return on same side of radiator is that, particularly on high water volume radiators, the hot water will settle across the top first.
If there is air, the water will splash about and not fully fill the radiator.
If fitting an air bleed valve as suggested does not cure problem, I have a private suspicion that you might need to close the bottom valve slightly in case water is running through too fast. Which makes little sense to conventional thinking, but, if you have to do this and it works, please do let me know.