The 'o' clip on the end of the clear pipe doesn't look like it's sitting on there properly. Or have you had to loosen it to take the gun apart.
Do you have a compressed air supply ? maybe you could blow air back down from the gun end to see if you have any leaks/blockages.
Are you definitely getting gas at the diffuser? If not there's a break in the pipe somewhere. Turn the pressure up release the feed roller pressure and trigger it to check it's getting there at the tip. It looks like you'll have to strip the lot down to check.