I have used Lightroom for years without experiencing the following.
I navigate to a folder containing CR2 files from a Canon S95, and I know I have done edits on some of the files in the past. In fact, the +/- indicator on the thumbnail indicates "Photo has Develop adjustments".
When I enter the Develop module, however, I am shown the original file, and the only History indicates the date the file was imported.
Most of the files in the folder have adjustments made, and the History indicates those adjustments, and the modified file is visible.
Any thoughts on how to rectify these files that Lightroom seems to know have been adjusted in the past, but those changes appear lost in the Develop module?
Just to clarify, have you opened the same catalog in which you made the adjustments to those images previously?
yes. FYI, when I look in the folder that contains the image in the Finder, there is a corresponding XMP file.
Then try right clicking on the file and choosing Metadata->Read metadata from file. I don't use XMP files, but this might help.