Four usual suspects, in order of likelihood: tip dry (paint crusting on the needle — wipe it with a damp swab), paint too thick for your pressure (thin toward skim milk or raise PSI a few pounds), water in the airline (drain the moisture trap), or a loose nozzle/needle seating letting air bubble back into the cup.
If spitting starts mid-session after spraying fine, it is almost always tip dry. If it spits from the first trigger pull, check thinning and seating first.
Go deeper in our airbrushes guides.