I've never had the model of incubator you do, but when you replace a wafer you have to readjust the calibration. Each wafer is different, even if rated for the same temperature.
The wafer normally screws onto a long screw threaded through a bracket, and a microswitch is also mounted to the bracket.
There are two different adjustment styles, one that turns the screw attached to the wafer, and the other moves the microswitch relative to the wafer. Whichever style it is, be sure the wafer is securely screwed to its mounting screw, and adjust the adjustable side until the microswitch is off. Then adjust it so that the microswitch just comes on, and let the incubator run until it reaches its set temperature. Adjust again just a tiny amount until you reach the setting that maintains your desired average temperature. Then lock it down, snugly but not too tight.