Check the nozzle is clean, with a magnifying glass you should be able to see the sheen of the metal right down to the tip of the nozzle, if it looks like a light at the end of a tunnel, you prob have some paint build up in there.
I also had air leaking in the head, if you run a wet paint brush round the seams while spraying air, if there's a leak you will see bubbles appear, I sealed mine with ptfe tape, and since then it's been running alot smoother.
Bent needle or damaged nozzle can also cause problems.