That's what my problem was. I didn't have any thermometers in mine and depended on the built in one completely. I couldn't get the hydrometer to read over 50%. I'm prepared and ready for the next time. I bought 3 thermometers/hydrometers to put in there. I'm going to place them in various areas at egg level. I think that if your going to trust the built in digital system you might as well just throw the eggs away. I had no luck at all.
One day the thermometer I had on the inside read lower and I tried to turn the temp up and It would go below 38 but not over 38. I guess I never figured there was a way of just calibrating 38 to where you wanted it to be. The instructions are incomprehensible. I ordered another thermometer/ hygrometer for the inside. That made me feel better. I've got eggs I started yesterday in there now. These are shipped eggs. I'm adding no water this time till lock down. Been warmer and very rainy. And hygrometer shows 35%. I think it may run low, so I'm happy not worrying about adding water. That is unless the air cells look too big at 7 or 14 days. Also, since these eggs were shipped and there were 15 of them, I cut up an egg carton took out the bottom of each cell and set them with air cell up. I am intermittently tilting the whole incubator. Only slightly at first but I plan to tilt further as I am hoping the air cells stay put.