It may be caused by wrong size of the cup holder and the tube is held in the cup holder. too tightly. The cup holder size judgment: the tube should be fully held in the cup holder, but when sealing the tail, the cup holder should not affect the tube shape’s.natural change

It may be caused by too small heating head. Just replace it with bigger heating head. How to judge the correct size of the heating head: Insert the heating head into the tube, then pull out, it will be fine if you can feel a slight suction when pulling out.