Are you putting water in a trough? Is there several troughs with some smaller than others? Humidity is directly proportional to surface area of water. I ask how and where your putting the water because if it's the same trough all the time then yes, that same surface area if covered by a mm of water or inch of water will result in same humidity until it starts to dry out. Depth of water in any given container only means time before it dries up. It's the surface area of water that controls humidity. If the incubator has different size bottom troughs then try the smallest one only and see what that does. For example, I live in New England and hatch time is wet spring. For my incubator in this environment a shot glass of water sitting in it will raise the RH to mid to high thirties. My goal is 30% RH during the first 18 days of incubation. To average this I let the water dry out and run incubator dry for a day before filling shot glass again. This spring that means the RH is fluctuating from 15% to 38% which averages darn close to my target. Around day 14 I'll candle and make a final check of air cell size, if still too small will run dry until day 18 when humidity is put to 70-75% RH. Or if air cells are on large size will keep the shot glass full until day 18 to retard the air cell growth. If you ever want to stop the growth completely just up to hatching humidity for remainder of incubation. At 70% RH eggs wont lose any moisture.
