@duncangray, that sounds like a combination of permissions & 32 vs 64-bit context-menu ... crapola. (To me that's a good way of putting it.)
Oh, you can run into issues permission-wise if you were running Everything "As Administrator", yet your application (say mp3 player) was running as a Limited user.
And then 32 vs 64-bit. (Mostly) a 32-bit applications context menu items will only show in another 32-bit application, like Everything x86. (Mostly) a 64-bit applications context menu items will only show in another 64-bit application like Everything x64. In some cases a particular program provides some sort of work-around so that its context menu items are seen both 32 & 64-bit versions of another program. Sometimes you would need to install both 32 & 64-bit versions of a particular program for its context menu's to show up in the respective version of Everything (or some other program).
Another issue that can crop up, is a "too long path" issue. In general, Everything (Windows ?) is good for ~xxx character pathnames, but other applications may be limited to ~260.
(And while we're here, I'll just throw this in,
You cannot delete a file or a folder on an NTFS file system volume and also
Naming Files, Paths, and Namespaces.)