What are your water parameters? Lighting schedule change?
Are you seeing the flesh recede or is it just closed? Duncans are divas and will go into hiding for days and then all of a sudden pop back out with new head buds.
Duncans are one of my favorite corals but they can definitely be temperamental. Are you spot feeding it? I wouldn't be concerned considering the new growths. Mine's grown 10-12 new heads in the year since I've had it.