Can't unscrew the cap of the water well on my steam iron?

I have a Team International steam iron with a screw cap on the water well. When the iron is heated up and the water is hot, this screw cap locks so you cannot open the well and risk injury from the steam in the well. It is supposed to be able to open again when it has cooled down.

Now however, the screw is still locked even though the iron is cold. The screw top turns half a turn, before appearing to hit some sort of catch on the inside of the well. I have tried lifitng as I screw, shaking the well, holding upside down etc. Any ideas?