does it make any sound while you press the button? if so, get someone to press down on the edge of the flap while you press the button.

My coupe did this and this fixed it. small cable to hold screwcap on had got jammed

 

Im writing this because i had the same problem but found no solution on the internet. But basically this problem is almost always caused by the locking motor malfunctioning, this motor has a mechanism that gives it an extended resting position, so when it stops working the fuel cap door will stay locked. There is no documentation on any emergency fuel cap opening (at least on my model 508, 2011 automatic diesel) but the solution is very simple. Inside the wheel house on the fuel door side there is a dirt cover, you should be able to loosen this with a flathead pretty easily. Reach your hand up under the cover and locate the motor, when you get a hold of it, twist it inward and it should pop right out along with the fuel lid opening. Hope this helps someone in the future.

 

Thank you for this, I have just applied this technique and it worked.