I recently ran across a problem with the latest release of Lightroom CC (2015.1) where my NVidia 660M stopped working in LR, but was active in PS, AI and DW.
For anyone else that runs across this problem, I opened the NVidia control panel, selected the "Manage 3D settings" link, selected Lightroom as the "Program to Customize", and set a number of fields to "Application Controlled". Not all of the fields have this option, but a few do.
After doing this, LR once again recognized my card (after restarting LR, of course).
I hope this helps!