First, place the pump towards the water surface. If it is lowered, the pressure will be higher and air will not enter, so the bubbles will not blow.
Look at the part where you insert the bubble tube and connect it to the pump. It's a small plug that can be removed, turn it another direction and try it.
It is also possible that the pump is clogged with dirt, just open it and clean it.
The oxygen pump motor is lost, resulting in insufficient pressure. Shorten the tube length appropriately or do not place the gas head too deep.
The hose may also be kinked, preventing air from escaping. It depends if the oxygen pump is still working. If water is still flowing from the other end while it is running, this means that your oxygen pump body needs to be changed and the plastic tube connected to the outside doit be reinserted.
If the oxygen pump stops working, you must disassemble it and clean the inside of the pump body. If it still does not work, the oxygen pump motor may be burned out.