The older type screw out, get a little wrench on & take the fitting out. See if grease flows through the fitting when it is out. Probably full of dirt & dry grease.
Take a little wire and scratch in the hole where the zerk was. Probably dirt, rust, dry grease, scratch it out. Can spray in a little penitrating oil, can help loosen up dirt & rust and dry oil around the whole joint.
Put back together and grease.
On my plows, often there is too much weight or pressure on some of the zerks and can't get any grease in. When the plow is sitting on the ground, or in the ground, the pressures are at different points and those zerks will work fine then. If they are in a bind, try greasing when the plow is down, or jacked up, or whatever.
--->Paul