Dirt in the foot valve would do it for sure. You could add a check valve farther up to solve the problem.

marshmelloman, with every positive intention I urge you to verify that the foot valve is correctly installed. You do not say what kind of hose you have used for the 'suction' end. I would use PVC pipe, designated suction hose, or regular garden hose with descending order of confidence. Persist with what you have in place but try to put a 'wicker basket' type of device around the foot valve to keep debris out but still allow enough quantity of water for the pump to draw without laboring. The sound of the motor will tell you when this is happening.
Also see if you can have a way of 'floating' the foot valve end of the suction hose. This will keep the foot valve off the bottom and reduce the 'head' against which the pump has to work to maintain its prime. I have seen this done with a rope and pulley system.
And that, my friend, is the most I can help you with on this fine day. Someone else will have to take it from here if you still have the problem.