You may not be getting enough pressure to the tank. Please make sure that the nipple on the tank is clear. Also make sure the line from the muffler to the tank is clear, with no kinks or restrictions of any kind. Do the same with the tubing in the tank. You may also need to see how long your fuel lines are. Shorter is better. If the tank's too far aft, there's a problem.

What size tank? Shape? Larger than 10 oz. is too much, and can cause fuel draw problems. Some shapes just pick up vibration better is some models. Make sure the tank is well-insulated against vibration...in some flight regimes, vibration can change enough to excite the fuel in the tank to pick up air.