Piaggio Zip 50cc 2-stroke oil not flowing to the carb?

My Son's Zip won't start and it looks like the 2-stroke oil is not flowing to the carb. There is plenty in the tank and when I take the pipe off at the bottom of the oil tank it flows freely. I thought this was a gravity feed but happy to be corrected and advised of a possible fault. It has been getting used through the winter but not much.
Any thoughts/direction will be much appreciated. We need to keep it going till June when he turns 17 and gets a 125.