Diesel fuel filler cap problem

Hi all. Apologies if I'm posting in the wrong room. I have a Ravenna Autosleeper but the query could relate to any vehicle. I have a screw on type fuel cap. When I put it to the fuel neck I can screw it on until there is a resistance and any further twisting results in a clicking and rotation while not getting tighter. If I then turn the key a quarter turn in the lock I would expect it to free spin in the locked position, but it simply unscrews and is back in my hand. I've tried the same with the key in both possible positions before repeating the above. 
Anyone got any idea if I'm doing something wrong before I assume the cap is broken? Thanks in advance. Best regards Rich

My petrol one has gone that way. it just acts like a non-lockable screw on one.
I was going to buy a new one buy I haven't got a round tuit yet. It's passed two MOTs like that.

Frank