Make sure that you do not have some dirt or debris in the actual nettle valve. This has caused me problems in the past. The engine runs great for awhile then quits after a couple of minutes into the flight.

Disconnect the fuel line on the valve from the tank. Then take the nettle valve out. Get a long piece of fuel tubing and put it on the outlet running to the engine.

Place your finger over the fuel intake on the nettle valve assembly and suck on the fuel line attached to the outlet going to the engine. If there is any debris in the valve this will usually free it. Be careful not to choke yourself.

Once you have done this replace the nettle valve. Take a piece of fuel line and attach it to the inlet on the valve that comes from the tank. With the valve open you should be able to easily blow air through the valve.

Once you get your problem resolved put a filter in the line coming from the tank.

Hope this helps.