I have a Nature's Spirit incubator that I've been using a couple seasons now. Previously, the egg tubs have held about a one degree difference between what the thermostat was set at. I've never had more than one tub incubating at a time before. I now have 4 tubs and have to keep the temp set at 94 or 95 to keep the tub temp at 89. Also, there is a temp difference between 88 and 91 in different tubs.

Is this normal and how do you handle keeping one tub from going too high while trying to keep others from going too low?