Hard to say...does the pump turn on and off correctly if you manually activate the float? Is the water actually rising to the point where it activates the float? or does it turn on while the water is lower (defective float switch?)? How deep is the sump? How far from the bottom of the sump does the pump sit? Do you have a check valve on the outlet (this keeps most of the water you just pumped out from falling back into the sump from the veritcal portion of the pipe)? Ground water for an area can change if there is construction or with long-term weather patterns. Plus, if there is a particularly big rainstorm or snowmelt. These actions can sometimes take awhile before they filter down and affect the water in the sump.