The stiffness of a pair of pliers can have several causes: Gummed up oil can be one cause. If a pair of pliers is rarely used and is therefore never or rarely oiled, it will "seize up". A "joint flush" with thin oil can help.
Spray the thin oil on the joint and now move the pliers legs back and forth briskly at least 30-50 times. Black, brown old oil should come out on the running side of the rivet and the pliers should run more smoothly again.
Then, after thoroughly cleaning off, another drop of thick oil (e.g. any motor oil) should be applied to the joint.