Your lips and mouth muscles are too stiff to pucker deep into the pump. Try to practice puckering 10 times before using the pump. Pucker your lips as far out as you can. This action works 34 facial muscles. Do lip yoga or lip gymnastic before your attempts.
You must have complete contact—meaning air-tight between the lip-shaper mouthpiece and your mouth. If you cannot use the pump hands-free at first, hold it until you learn to use it hands-free.
Your pump might be too large for you. In this case, use the mouth cover to help you create a tight-seal between your mouth and the pump.
Your pump may be too small for you. Remember to buy the right pump for your lip width.
The pump may have an air-leakage issue. A good way to check if your lip pump is working properly is to test it on your upper thigh! Compress the lip pump to its fullest capacity. Then place it on your upper thigh and release the lip pump to create a local suction. If you can create a suction on your upper thigh, you should be able to do the same on your lips. Your thigh will not swell up as your lips but you might get a cupping mark there.
If your lips are non-responsive or resistant to plumping, try 1 lip at a time.
If it is difficult for you to create a good suction after you add a mouth cushion, try to swallow some air while you plump.