This seems to be quite a common question “I am just writing to you because I have been using this pump and it has completely stopped working! It still has power but no suction at all and no ability to turn it up - it was starting to have periods of not working and now it has completely stopped.”
First, you need to check the tiny white membrane disc. This tiny parts controls the whole suction and performance of the pump, and it needs to have no rips, tears, or dry milk residue. It has to be in perfect condition. They do get wear’n’tear, so have you checked that part? We do have spares available here. Change this part over and tell me if your pump works!