Hi Monica,
It seems this issue is already discovered and fixed for up coming patches and release. When you have Group layers created it may miss some of the layers in UI controls like Cell Value Tool.
This fix should be available soon in patch 16.0.3 and next release.
You may try following workaround till then,
In cell value tool expand options -> Select neighbouring cells tab -> Now you could switch between raster through select raster list box. ( I know it may be little annoying).
Regards,
mitesh