My guess first is that, being it is high current, the hammer is getting welded to the anvil a little. generally i can tap the outer casing and it will brake loose. possible solution: use graphite or some lubricant??
Yes I'm 99.9% sure that it's due to contact welding, which is a common problem with high current contacts (whether in relays or regular switches).
Some relays (and switches) counter it by letting one contact slide over the contact when touching ( a sort of "L" shaped movement), but the best is getting relays with wolfram (tungsten) contacts, as they're very hard to affect (I have seen slight pitting in wolfram contacts, but never experienced them welding).
A snubber network over each contact pair is yet a method (as long as the contacts are rated for the current you're switching), as this kills the high voltage inductive spark that makes the welding happen.
If used unipolar, a fast diode could be used.