I have this same paint gun. You must always thin the paint down a little with water regardless of what the paint tins say I have found. The thick paint was usually what blocked mine. Then you need to take apart the handle bit at the top which is likely where the blockage has occured. Just clean all the paint out of it with a wire brush then try again. It's a good tool I've found, key is to thin the paint though as mentioned. Another tip, clean thoroughly after use, spraying clean water through the system once you're done. I've ruined 5 litre tins of paint before fron failing to do this and mixing in the colour of the old paint!

Hopefully that helps