Just be patient and they should open more in no time.
From my own experience, the branching types of dendrophyllia (smaller heads, longer stalks) are a bit more "shy" than the short stalked/large head types. I had mine do the same thing when I first got them and they would only extend when feeding and then close up again when there is no more smell of food. But after some time of just feeding them regularly at the same time each day, they pretty much open by themselves even w/o food and eventually almost all day.
Also, from what I have seen on mine, the growth pattern (forming of new heads/babies) on this type is similar to a black tubastrea. The new heads can grow out facing in all directions. Right now, it should not be a problem for you since you still have it attached to a plug. But once it becomes settled and you don't have to take it out for feeding anymore, make sure you affix it in an area of the tank where you have enough space to "maneuver" while feeding all the heads. I have the main stalk on mine facing upwards so I can feed all of the heads all around and hopefully any new ones which will grow.