Maybe a bad reed valve.
Go to this link to see how a Reed valve works. It basically should create a vacuum on the downstroke to cause the gas to be sucked into the chanber. If it is defective it will not create the vacuum to keep sucking in the gas.
It could be the reed valve or a own head gasket or defective engine gasket seals. Any of these would cause an air leak that would prevent a vaccum from being created.