Paint build up

Saul,
There are several reasons this can be happening, but the most obvious are:
1) The paint is not thinned properly to match the needle and/or pressure
2) The needle being used is not compatible with the medium being used
3) The needle is dirty or clogged
4) The compressor's regulator is set improperly to match the paint
5) The gun's internals are dirty (if it's an internal mix unit)
Some build up of paint on the tip may be ok, but if it's sufficient to hiner the gun's proper operation, something isn't right.
Glenn

Saul,
Acrylics will dry fast when exposed to air.
Some faster than others. As you airbrush some paint
dries on the tip and keeps building up until it's cleaned
off. Keep some of the paint thinner handy and something
soft to clean the tip with .
Clean the tip , point the airbrush away
from your model spray to ensure tip is free of dried paint,
thinner etc.(don't want to ruin the paint job). Adjust paint
flow if needed, back to painting . Hope this helps.
Fuzzy

running acrilcs throughan airbrushes is fine until u clean it it can worse then enamel paint . so to save troaube use enamel paints instead of acrylics