How is the supply line from the tank plumbed? I ask because it sound like it's losing its prime.
Manufacturers ignore pump installation guidelines and place them way too far from the tank. My last TT had a horizontal run of about 15' and would not hold a prime.
I liken it to holding your finger on the end of a straw full of water and then turning it horizontal. The water will run out.
I moved the pump directly over the tank and never had a problem again.
Other possibilities are that it has sucked debris up. If so, take the head of the pump apart and rinse all the parts off. Reassemble and it will be good.
Or it could be you have a leak someplace or the water heater is dripping from its TPO valve.