Removing a grease gun tip from a zerk is a different technique than attaching to the fitting. When attaching, one pushes the tip dead-on in order to make the seal. After applying the grease though, you should grasp the tip and pry it off with your hand if you are using a flex hose. If using a fixed fitting, then you can just pry the fitting off by moving the gun at an angle. I have the Lincoln pneumatic gun, and this works well for me. I believe most tips have a strong spring of some type that needs to grasp the zerk in order to seal tightly. A buildup of old grease will cause the tip to get stuck, as will a piece of fine grit.
May God speed your healing,
AB