Is what you have the Park GG1 Grease Gun? Basically it's a rebranded Duelco grease gun and these are prone to stoppages if an air bubble or air pocket in the grease gets to the top of the canister. Remove the top and see if there is a void. If so, push up from the bottom of the can with a screwdriver to fill in the open area, replace the top and push the plunger until grease flow resumes.