Unplug the sump pump. Bail out the sump pump to expose the pump's machinery.
Inspect the rod attached to the motor on the top of the sump pump. Move it up and down manually to see if it moves smoothly. Corrosion will keep the rod from moving smoothly. Clean the float road with steel wool.
Inspect the float ball. If it contains water, you need to replace it. Unscrew the float ball from the end of the float rod and screw on a replacement float ball.
Inspect your sump pump's check valve. It has an arrow on it. The arrow should be pointing away from the pump. If it's pointing towards the pump, remove and reinstall the check valve.
Locate the small weep hole between the check valve and the pump. Clean it out with a pipe cleaner.
Inspect the impeller, which is the fan-like piece that sits near the bottom of the pump's tank. If it is clogged by debris, then it will not turn properly. Unbolt the impeller and clean away any debris you find. Reinstall the impeller with new gaskets and sealant. Locate and replace the mesh filter in the sump pump, since only a damaged or missing filter would allow debris to get near the impeller.
Below is troubleshooting work, web link, through to assist you in that.