Heavy condensation usually means that humidity has gone above 80% due to the chicks hatching and adding their own moisture in there. Open up vents to let some escape. If you have to, prop the corner of the bator on a pencil crossways for a few minutes to let some moisture escape so it comes back down-that's what I do when the windows fog up like that. They do have a hard time drying off when humidity is 80-90%.
Thanks for the tip!