Tony, i'm not sure that the paints you are using are made especially for AB'ing so just as a test, use some ink and have a go on some paper.
Are you using paints formulated for AB or are they just general acrylics that you are thinning down ? See how the ink goes if you can. We can tell then if your brushes are working ok as ink will flow through anything.
I know that some people here use tubed artist acrylics without any problem but they would have to pipe up as to how they get them to work. I haven't a clue as I always use AB paint OH apart from a tester pot of dulux emulsion from the hardware shop. You have to water it down a lot but i got it to work through my 0.3 brush.
Not sure about the tamiya stuff, only ever brushed it on with a hairy stick. I vaguely recall that they can be airbrushed but might have to be thinned with a tamiya thinner or reducer. I have no experience of them unfortunately. Hopefully one of the great members on here has used them with success.
This info probably wont help but i was just thinking about it and thought i'd post it up as a bit more information.
Lee