Just had this same issue. On closer inspection I found the pump motor was running, but since it wasn't pumping water, it was very quiet and you had to be close to feel the vibration. I had water at the inlet and the strainer was clean. Took the pump out and tore into it , but didn't see anything obvious wrong. Bought a better one and installed it and all was well. I decided to check closer and hooked the pump to my truck battery and felt for suction and found NONE. Took it apart again, this time removing the check valve cover and the pressure cover. Anyway the check valve was stuck closed. I pushed it open with my finger and put it all back together and got suction. Now I have a spare.
Point is check to see if the pump is actually running, It's not as noisy if it's not pumping.