Yes, most likely scenario is that the old grease has mixed with some dust or dried and hardened sufficiently that the grease gun won't push through the plug or seal it has created. If it has gotten to that stage, a full clean up as Jenkins suggests is definitely indicated - if it's that bad at the grease nipple, it could be worse around other parts of the joint, so best to be thorough and sure. Once cleaned, a light coating of grease on everything before reassembling, and then try the grease gun again. Hopefully, it won't be too difficult a fix, and shouldn't take too long to do. And then take the symptom as an indication that the joint probably hasn't been lubricated regularly enough in the past to keep that old grease moving out and being replaced with fresh. Good luck!