20 degrees would be a long way for the plug to turn on a spool pin. 5 to 10 would be more likely if this were the case. But, if the key functions normally, and you are picking the right direction, this *could* be the case: Perhaps your tension wrench is getting jammed against the inside-bottom of the shell. It's happened to me on occasion and threw me for a loop the first time.
Just a thought.
db
P.S. Don't pick locks you rely on.
