Can you temporarily run the discharge line to, say, a washtub or large bucket or bin? If you can, do that and HOLD the bucket down because it may 'run' away from you when the pump comes on, then have a 'friend' pour a good amount of water into the pit and wait for the pump to work. If it works fine, then your problem may be that the water is being siphoned back into the pit. You need to find out if the HEIGHT of your discharge line is too high. If it is, then water will be siphoned back. Remember, the pump runs just a little bit after the water is removed from the pit. Only long enought to 'insure' that the water is totally pushed through the discharge line. If the line is too HIGH, the pump doesn't know this, and it will still shut off.
Read the Installation Instructions for info or ctc. the Mfgr. Then, go and measure the total HEIGHT of the line. Beyond that, I can't hear it from here to tell you what it is.