Some iPhone 4S units cannot run iOS 5. These units shipped after iOS 6, and have changes to the hardware that iOS 5 has no drivers for.

It seems a good way to identify if the device is capable of running iOS 5 before attempting an install is to check the manufacture date on websites such as http://www.chipmunk.nl/cgi-fast/applemodel.cgi

If your device was manufactured before September 2012, it should run iOS 5 without any issue

If your device was manufactured after September 2012, you will likely run into issues with iOS 5.

If you were unable to use iOS 5 already, and were stuck at "Still waiting for root device", your device is unsupported by iOS 5.

This issue is not one that should be easy to fix. We will look into it at a later date. For now, the only way to run iOS 5 on a 4S is to get a device capable of running it.