The issue on early mk7 Golfs was generally the fuel flap actuator pin was catching on the part of the filler flap that it slots into when the car is locked. The actuator pin then didn’t (or couldn’t) retract far enough or not at all the next time the car was unlocked, preventing the flap from being opened.
As @Carbon VW has said, VW’s solution was to shave some of the plastic off the offending part on the inside of the filler flap to stop the actuator pin getting stuck.
It’s possible that if this problem is now happening for the first time on a 14 plate GTI, then it might be the fuel flap actuator that’s faulty and showing signs of failure, in which case it would need replacing.