Petrol filler flap

I have a 2008 Polo 1.2 "Match" and the petrol filler flap doesn't close properly, i.e. it won't latch.  So when I want to secure it shut, I have to hold it closed while locking the doors. But once the doors are unlocked, the flap springs open slightly. As we know, after refuelling the flap should close against spring pressure with a firm click - which mine doesn't.  I cannot see anything broken but don't know how the mechanism to hold it shut works.  Any helpful advice would be much appreciated. Thanks in advance.