I have posted a couple of times about using nerite snails in cichlid tanks (search the site for those posts). If you aren’t familiar with them, let me add something that I didn’t mention previously. Though you may upright some snails that you find upside down, don’t expect them to always start moving immediately. In fact, sometimes you’ll see an upside down snail and the aperture will look empty. Neither means the snail is dead.
I suspect the disappearance of the snail from the aperture (i.e., it appears the snail is no longer in the shell) is that it often retracts into the shell body as a defense mechanism. If the snail is upside down and is unable to right itself, continuing to stay in the aperture makes it defenseless. Also, it can often take time (sometimes hours) before the snail will emerge after being returned to its upright position. I have found snails upside down before, righted them, and then several hours later noticed that they haven’t moved. Give them time. More often than not, I will check on them the next day and they’ve moved along.
If you find one upside down, you right it, and 24 hours later it hasn’t moved, it is probably dead and should be removed. I don’t believe I’ve ever had one remain stationary more than 24 hours after being righted.