Depends on the grease in the Dualco pump gun. Sometimes a heavy grease will jamb up the tip of the gun, requiring removal of the tip and cleaning it completely. High voc liquids and some compressed air will aid in the process. As HillRider suggests this grease gun works on a vacuum system from the grease cylinder and air voids will stop the mechanism. In a few rare cases I have seen the guns fail altogether due to seal leakage and wear on the piston. HTH, MH