try taking grease nipple out does grease go thru it when connected to grease gun if does nothing wrong with nipple, one thing u can do with a worn nipple is put a rag over nipple then gun on top of that this can help to tighten the seal get a decent gun 7 000 psi one if u can i dont see any thing wrong with leaving a hairdryer on joint for ten mins worth a go might get grease moving try spraying degreaser into nipple hole as long as u purge with fresh grease a couple of times will be better than what was in it before , if all else fails strip down time:D wanner grease guns swiss made v good secondhand ones on ebay for not a lot od money