I was wondering if when you turn the pump on, does it kick itself back off? If it does, there's a problem with the pump motor. The internal overloads would shut the motor off.
If it doesn't turn itself off, then there is probably something blocking the inlet to the impeller and will need to be cleaned. I'm assuming the pump is inside a sealed sump. Open it up and the pump should be fairly easy to remove. While you have it out, look at the pipe right above the pump discharge and below the checkvalve. The could be a small hole in the pipe. Make sure it isn't plugged up. The hole is there to prevent the pump from getting airlocked. (That itself could be the problem,too).