Old grease can get pretty think, Actually it can get down right hard.
So a pin that has not been greased in a while and will not take more grease can be caused by the grease hardening.
The problem with forcing grease through with the hammer devices, is that the grease will find the path of least resistance and not grease around the entire pin leaving most of the pin lubricated by the old hard grease.
The only correct way to cure this problem is to remove the pin, clean it and the grease orifices and replace. Then grease regularly.
I have never had a regularly greased grease fitting stop taking grease.
JMHO