The same thing happened to my loppers when I tried to bite a bit more than they could chew, nothing thicker than your thumb, I seem to remember being told (the rule of thumb) to late damage done!.
I just use them fully extended now, at least they still work.
With mine,one of the arms had bent slightly and went a little oval and preventing the arm sliding back. If this is the problem, I guess you could try to make it round again by putting it in a vice but you would have to be sooo precise.