I added a fan to mine, but successfully hatched before that. I set the thermometer on top of the eggs in the middle of the batch. The temp was very stable. Maybe you need a new wafer.

incubator top by jenjscott, on Flickr
Here's the inside you can see the two small computer fans I've installed inside the hardware cloth next to the thermostat unit.

incubator inside by jenjscott, on Flickr

incubator top by jenjscott, on Flickr
Here's the inside you can see the two small computer fans I've installed inside the hardware cloth next to the thermostat unit.

incubator inside by jenjscott, on Flickr