Game version
Major version changes can alter how data is stored or interpreted. When possible, know which version created the save and which version will open it.
Platform
A save created on one platform may not be directly interchangeable with another. Platform-specific paths, formats, or features can affect compatibility.
Installation type
Different installation methods can use different storage locations. A valid save can appear to be missing simply because the destination application is looking somewhere else.
File integrity
A partial copy or damaged file should never replace the original working backup. Compare copies when something looks unexpectedly different.
Use a controlled test
Keep one untouched backup, make one working copy, and test that copy. This makes it much easier to identify the cause if the save fails.