I had several on my loader and mower deck that would not accept grease. I pulled them out, and most of them had paint over the little ball in the middle of the zerk. After cleaning, all but 2 would take grease. I replaced the other 2 with new ones, and they all work fine now. I went to an auto parts store, and found that they had 2 sizes, mine were the larger size. You can use a sharp knife blade to chip the paint away from the ball and face of the zerk.
Joe W.