Fuel filler flap will not unlock after central locking has been undone

If the fuel door usually pops open with a fuel door switch or pull mechanism, have somone switch it on and hold it.
While they are doing that, get something that will fit between the seems of the fuel door and not scratch it. Cable ties are good for this as they are plastic and have the tab at one end to pull on the door. Once you have the tool to get behind the fuel door, pull on it to see if it will open.

Generally this only works on fuel doors that open on their own by aid of a spring when a switch or open fuel door lever is actuated.

Since it is automatic, I am assuming this is the case as any other setup is opened by hand at the door.

If you have to a paper clip will work. Straighten it out and put a 90 degree bend at one end. Insert the 90 degree end in first and get it behind the fuel door. Twist it so it can pull on the door then it should come open.

More than likely the spring that opens the door is gone and without it, the piston that locks the door will work but there is nothing to push the door open.

I hope this information helps.