check the nozzle on the gun, it might just have something partially blocking it.
they have alot of problems with the unloaders, the valve that puts them into bypass when you let off the trigger. Sometimes if they are weak a check-valve could have something in it not allowing it to close fully. there usually around six check-valves 3 input and 3 output. get some pump saver (home depot sells it in a aerosol can, and see if it helps. otherwise pull it apart and see if a valve is messed up.