I've never seen/had one of these but it is more or less the same set up as the Tell 2, which I had similar problems with. With the Tell, I think the plate holding the breech washer was slightly bent backwards, and I didn't fancy trying to bend it back, but a thicker breech washer that was well lubed solved it.
Does the hooky thing feel snug when it locks onto the pin or does it feel sloppy? If it feels fairly snug, then I'd try for the breech seal. If it feels a bit sloppy, then inspect the pin it locks up on. On the Tell's, I have seen these bent which causes the gun to not lock up right. If the pin seems straight, then you could bulid up the hooky thing with weld a touch and then file it to suit, making sure it was not too tight and causing the pin to bend.
If everything seems ok and a new breech washer doesn't cure it, then look at the washers in the cylinder.