Sometimes the grease cakes up behind the zerk fitting. I have taken an awl or "small" flat nose punch and gently tapped the ball portion of the zerk. Hopefully this will loosen up the zerk enough so the grease that is under pressure will begin to flow. Most of the time the pressurize grease will force out the caked up grease. The worst cse is you may have to replace a zerk either new or an old working one from an old u joint etc.
JUS' MY $0.02 WORTH
JUS' MY $0.02 WORTH