Very useful information.Humidity too low. Calibrate your hygrometer with a salt test. They change readings over time and are not accurate when new anyway. Reasons for changing over time could be gunk getting into them, difference in battery power, etc. As long as you calibrate before each hatching season you'll always know true RH. I use to hatch at 65% RH and that was too low unless you pulled chicks out as they hatch. Reason being is if left in with wet shells RH gets so high the album turns to glue and when RH gets back under 70% chicks stick solid in shell. Last time at 65% had hatch where most pipped overnight with many hatching, awoke to all non hatched and piped were stuck. 70%+ for me anymore. You can look up salt test on the BYC search or google. Here's a quick and easy of it: Fill a milk cap or similar size cap/container with salt. Add drops of water until saturated. I poor off any standing water. Put cap and hygrometer into a sealed container. I use a quart zip seal bag and allow for pillow of air. Wait at least 4 hours and 6 usually is enough for readings to stabilize. Basically wait an hour to see if it's still climbing before stopping test. A salt environment is 75% RH nearly exactly for all normal temp ranges. Minus your reading from 75 for calibration number. Ex. your reading is 87: 75-87= -12. Write -12 on masking tape and afix to incubator as reminder to subtract 12 from all your readings for true RH.
