I've had a yellow polyped sarcophyton for about 9 months now. It goes through "stages" of dormancy and activity. My sarco enjoys moderate flow (tail edge of 620 gph). They need a good flow to wash off their mucus mantle when they clean up the crown. You'll see a shiny layer appear on the sarco's crown when it is in the "cleaning" mode. It will also bend over (limp) and appear to be dying on you. That is okay.
My sarco also tends to lean out towards the front of the tank after the tank light is off. It is leaning towards the ambient light of the room it is in (living room), so nothing to worry about there either.
The sarcophyton is a very active coral and will react to just about any change inside the tank. Mine has endured top-off overflow (low salinity), high nitrates, high temperature (over 80F), and removal from the water.
If you can be consistent with the sarco, it will open again. Just make sure your water temperature is between 75 F and 80 F. When my tank goes above 80F, the sarco usually closes up and only shows its polyps for a few hours in the morning.
Check your lighting fixture. If they are older than 6 months, get new bulbs. The sarco is a photosynthetic coral and is very sensitive to strong lighting. Don't try to give it super-high-output lighting as that will damage it.
Visit
www.wetwebmedia.com for additional information on the sarcophyton corals. That's where I learned most of my how-to. Like you, I was concerned about my sarco when I first got it and it was always limp. Then I learned, with experience, that it can take up to 7 days for the sarco to "recover" from anything that disturbs it.
Anecdotally, my starry blenny loves to sit in the crown of my sarcophyton, especially when the polyps are out. You might find that a denizen of your reef is also resting on your leather's crown, which will make it close up for a day or less.