All Righty ... What are you doing today ?
I, too, feel your pain, James. I have that problem with regularity- I strip the ab down to parts and run it through my little sonic cleaner in a solution of Simple Green soap and warm water. After that, a close inspection and a fresh vibro-bath for any parts that need it. I reassemble the brush, using Airbrush Lube for any pressure- or paint-bearing joints and it's back to it.
I wonder, though, if you have a bad o-ring somewhere. I recently replaced the one closet to the tip in mine recently and performance is way up.
Also, I have taken to unscrewing the tiny, delicate nose tip and pulling the needle through the front of the brush (after flushing with thinner) to get that last, nasty bit of paint out. I'm finding I get much less binding and such. Finally, if you're using both water- and solvent-based paints, take a moment to do this. It will avoid having traces of paint jellied up by the wring thinner.
It sounds a bit fussy, but fight fire with fire, as they say!
Cheers-
chuk