Either the nipple is seized or there's too much muck packed inside the channel. Buy some replacement nipples: in straight, 45 and 90 degree inlets. Take the old one out, and to test it run a bit of grease through it. If it flows well, the problem is deeper. That being the case, you can usually clear the mess just inside there with a pick. An additional measure is to soften the muck a bit with a propane torch. Heat it gently. These measures have rarely failed. When you're done, grease it more often. That will keep things happier in there.
