Follow the advice on checking the fuel system. Be aware tiny breaks or cuts usually near the tubing ends are HARD to see. LOOK CLOSELY! listen to see if the clunk rattles as it should.
A sharp thrust of the plane rearward will often get it right. Be sure the tank is not foaming.
Foam on the outside to isolate vibration is the key to preventing foaming on the inside.
Try this old article. I printed it out and carry a copy or two in my field box.

http://www.smrcc.net/Tips/Engine/2-stroke_engine.html