i have had similar issues with cheap or worn guns. you can buy just the end or even a hose with a new end for cheap.
sometimes the zirks are just flat plugged and need replaced or cleaned, but from your description it sounds like your gun isnt making a positive contact.
in a pinch, you can do some interesting manuevers by holding the gun tight to the zirk with one hand and using body parts to support the tube while pumping with the free hand. chest, belly, thighs, etc. been there, done that, prior to getting new fittings for the grease gun.