About 20 months ago I made the same mistake of buying a goniopora. Then it had 3 full heads with nice extension.
I mounted it high in my aquarium, figuring more intense light would be good for it to produce energy from the zooxanthellae. It kept getting knocked down by hermits, though, and eventually closed up tight for a couple of months at least.
Then I read where they are typically sand dwellers in lagoons, where the water is higher in nutrients. So, when I moved my 30 gallon into the 46 bowfront, I placed it in the substrate in fairly light but noticeable current, away from everything else. This has been about 10 weeks or so. It now extends daily and has 23 polyps whereas when I first moved it, it only had 13.
BTW, this is a branched one, which are supposed to be easier to keep.