Grease and Shackles

I put on my shackles about a 2 months ago, I greased them well before I put them on but I alway had a little squeak. Now it is really bad, I went to re-grease them but the grease just oozes out around the zerk. I assume that they are full, but is it possible the zerk could be clogged, or is there a certain type of grease I show be using specific to this application?

Replace the fitting and use sythetic grease.