I helped my friend troubleshooting similar problem. here are my suggestions:
1. clean the leaves basket lid and close it tightly. then re-start the pump and inspect for air bubbles. you should see some little bubbles if there is air leaks. if you see some bubbles, you know the problem is air leak
2. find the air leak on intake pipe starting from the pond edge to your pump
3. if you can't see any air leak, swap this pump with the other pump since you have 3 identical pumps. if the problem follows, you know it's the pump. if the problem didn't follow, start digging. your intake pipe has an air leak somewhere.
good luck
Steve