Does the key still rotate the plug?
IIRC, they rotate to about 45 degrees and then open. It's possible that the shackle and/or body has corroded and/or rusted (brass bodies, steel shackles) and it is no longer freely opening. Try using a penetrating lubricant and small taps on the side of the body and shackle to try to free it up (make sure the key is rotated during this process, obviously). Ball bearings that retain the shackle might be stuck too, but the same process should remedy that.
Worst case scenario, put the shackle in a vise and use a pry bar to try to free it from the body WHILE rotating the key.
They come apart very easily and are just as easy to service.