Dirt or grime has found its way inside the fitting. If you can remove the fitting and soak it in a degreaser, or use a small pick and try and push open the zerk ball several times.
Sometimes the outlet hole where the grease goes through the zerk and into the bushing gets an obstruction against it. Sometimes it's pressure from the bushing or it's old dried grease. Worst case scenario is that it has rusted shut. My Ih dump has several on the leaf springs that no longer take grease and I don't like it.
When I first got my tractor, one of the grease fittings on the loader wouldn't take grease. There is a grease groove on the pivot pin, and it was turned 180 from the zerk. Tapping the pivot out enough to rotate it to allow the groove to face the zerk took care of my problem.