Leave it be. It will decide where it wants to be. While you can kind of get them in the general area you want them, they will choose their spot.
The only reasons I can see that it would stay under the rock and die would be it was already dying, aclimation shock, not enough lighting, less than perfect water quality.
Could you give us some more input about your tank? How old, what type of lighting, water parameters, other inhabitants, etc.
If you have not already done so, now would be the time to cover all powerheads with netting. A roaming nem is more than likely to end up in the powerhead.