Needle blockage: when the needle is blocked, the liquid medicine in the syringe cannot flow into the human body smoothly, so the needle cannot be ejected.

Damage to syringe: If the syringe is damaged, such as the joint between the syringe rod and the needle is broken or the syringe rod is deformed, the needle cannot be ejected. In this case, even if the liquid medicine has been injected into the human body, the needle can not rebound normally.