Ours are patchy. Some are quite hot, some tepid but venting them produces water immediately so it isn't an air lock problew. I have read that one can 'balance' them but you need to know the order they are installed in and work your way sytematically around them and you need to be able to measure a 12 degree difference between the inlet and outlet... Ooer!
Presumably you get a pro in but has anyone done this?