Is it changing naturally from the changes in the environment or because of changes inside the incubator? Are you running with water or dry? What does your incubator run with no water?
Humidity has it's biggest effect over the incubation period. If it is staying high more often than not and the eggs aren't loosing the moisture they should you have a higher risk of problems at hatch. If it's staying down toward the 30-40 the majority with shorter periods of spiking and the eggs are loosing the appropriet moisture, then there's less chance of problems. If mine stays above 25% dry, I run dry and just keep an eye on my air cells to make sure they don't get too big too fast. If it is dry and the bator can't maintain enough humidity I add a wet sponge and it usually holds it right around 30%. I can always tell when my sponge is drying out because I don't have temperature control (LG9200) and my temp starts increasing, so I re wet it, usually about once a day.