You probably have a suction side actuator that has failed, or quit working. This usually happens when you use the spa. Both actuators rotate correctly, you use the spa with no issues, then when you are done, you put the system back into pool mode and only the return side rotates back to pool. You don't know that the other actuator has not rotated until the next morning or maybe later, if you don't look at the pool often.
This is easy enough to check. Place the system into spa mode and check to see if both actuators are rotating. Just be sure that you can get out to the equipment to see this after pushing the spa button. Most systems will also let you put it into spa mode right at the equipment, however you must place the system into service mode first, with the exception of the Easytouch system.