1. It could be you still have dried paint somewhere in the works. Try soaking your 'brush in a mineral spirits bath for several hours, then try taking it apart.
2. Check the bottle your are spraying from to ensure paint hasn't clogged the siphon hose or tip where it attaches into the 'brush. I have had good success with cleaning nozzles, hoses, etc. with pipe cleaners. Cut 'em in half, put a few drops of thinner on one end and go to it.
3. Cleaning your 'brush with a mineral spirits thinner will add a little bit of lubrication that laquer thinner won't...unless you're spraying lacquer-based paints. You might want to consider changing. Lacquer thinner has the potential to be a bit more toxic, also.
4. Consider calling the company and getting their take on the problem.
Gip Winecoff