I replaced the u joints on my truck. They went on pretty easy. I though the project was almost over until I went to grease the u joints before reinstalling the driveshaft. I can't get any of the 3 u joints to take grease. I am also using a new grease gun that has grease coming out but doesn't seem to lock to the zerk. Is there something I need to do to the gun or zerks before pumping the grease?