Hi, it could be 2 possible reasons, youv'e got dried paint in you gun, this is the most likely as it's quite difficult to clean properly when you first start, what you need to do is take the gun apart(nozzle, needle), and soak in neat cleaning solution, standing the gun in the solution so it covers the well. This will proberly take a few hours or if youv'e got time overnight.When youv'e soaked it take the blunt end of the needle and scrape it down the inside of the nozzle this will take off loosened paint, clean the inside of the well with a stiff brush, and wipe the needle with a cloth. Now reasemble, run neat cleaning fluid through the gun and then plain water. You will now be ready to start again. Remember to flush with water between colours, diluted cleaner between clients, and soaked overnight in neat solution, a small film container is ideal to hold your nozzle pieces so you don't lose them. Now the second possible reason is youv'e cracked the nozzle, the only solution to this is to replace it. It's always a good idea to keep a spare nozzle anyway just in case, also if your gun is blocked and your in a hurry, you can always just swap the nozzles over untill youv'e got time to clean it properly.
Hope this helps you
Karen.xx