i had a similar problem when my mate gave me a load of cheap acrylics from his work they spray fine but as you pull back nothing seems to change then all of a sudden SPLAT!
its down to the tip drying out (least thats what it was with me) try a different paint (FW inks are good apart from white) or another trick is to nip to your local chemist (pharmacy if your in the u.s) and buy some glycerin or glycerol (dont quote me on the spellings here btw) thin it down with water then add small amounts to your paints it works as a retarder and stops the paint drying as fast giving you less tip drying
be carefull though if you add too much your paint will NEVER dry
