just picked up this Duncan with about 30 heads on it. I have it horizontal currently. All the heads have opened up every day but today only about half the heads are open. All the heads on the under side are being shy. Do I flip him vertical so they all face up? He's currently mid tank, low to moderate flow
I would try and rotate the colony to get all heads some light. Also what might be happening is that the bottom heads might be hitting the rock when they start to come out, which then they retract. I'm sure your water is just fine, as we can see SPS and a Cup Coral nearby are doing well. They do like some meaty spot feeding, just FYI.