There are way too many reasons as to why they might not open. Too much flow, water conditions, something agitating them, sediment on them, fungus- those are just a few. Have you checked your water quality? Are they new to your tank- they may just need to get used to it if they are. I have had some close up for days to weeks and open up finally and some that never opened back up. How do the polyps look? plump and healthy? Or shriveled and brown?