Several questions.
What is the flow rate of the pump?
What is the overflow rating?
If the pump is too strong, the tank will overflow because the overflow cant keep up with draining the water back to the tank. You can add a ball valve inline with the pump to restric the flow a bit.
If the water is draining as fast as the pump is pumping, then you might not have enough water in the sump.
When the power is off, how full is your sump?
If it isnt within an inch or two of the top of the sump, you dont have enough water in the sump. While the power is off, fill up the tank until water starts flowing down the overflow, then full up the sump to within an inch or two of the top of the sump. Turn on the power to the pump, let it run for an hour or so, then make a mark on the sump where the water level is. This will be your fill line.
