Tell us about your incubator. What watt bulb are you using? When you say "only reaches"... so the light never goes out and that's as high as the temps get? Are you using a thermostat or dimmer?
You really need to get to 99-100 *average* temps... if you have a circulating fan.
If you are using a light bulb for the heat, try a larger wattage light bulb. You can also try wrapping the incubator in a blanket to get the heat up. Or, you might have to wait until Spring when temps are higher in the room where your incubator is....