The stock chains are junk, pure and simple.
You are better off getting a quality 415 or 41 chain from Tractor supply, Ace Hardware, ect.
The biggest reason why the stock chains have stiff links is due to bent side plates or poorly riveted link pins. You might be able to work the offending link(s) loose by prying the side plates apart a teeny little bit with a couple of screwdrivers, ect. If the problem is a bad link pin you will have to work the chain back and forth over and over until it wears loose.
Plan on replacing it soon anyways.