The usual solution for a problem like this is simply tap the side of the pin that takes grease smartly with a hammer while applying pressure to the zerk with a grease gun. This has also been the standard solution for U joint's that don't take grease in one cup.
In all the years i've been around farm equipment i'd hazard a guess that it works 98% of the time, with the remaining 2% being the pin or U joint in question had been ungreased for so long, disassembly and repair/replace was necessary.
If you physically can't get to the side of the pin or U joint to smack it with a hammer, in some cases a long bar can be used as sort of a punch to slide into limited space, then hammer the end of the bar.
You don't need to beat the problem area, just use sharp taps with the hammer to provide shock that usually displaces the forign matter blocking grease movement. The combination of that shock and hydrostatic pressure from the grease gun usually solves the problem.