There is a small but annoying bug if you try to apply the same profile to 2 or more images in the Develop module.
Say that you have 4 images: Images A & B in Profile 1, Images C & D in Profile 2. If you select all images in the Develop module (with Auto Sync on), and you choose the same profile for all 4 images, the profile will be changed only if this changes the profile for the most selected image. For instance, if image A or B is the most selected, and you choose Profile 1, then images C & D will not be changed. However, if image C or D is the most selected, then the new profile will be applied to all the images.
Lightroom seems to use the (incorrect) rule -- if the most selected image is already in the chosen profile, then there is nothing to change, but if the most selected image would be changed, then apply the change all the selected images.
This is an annoying bug because changes in color profile can be subtle, and it isn't always apparent that no change was made.
The work-arounds are obvious, once you are aware of the bug.