Pick one such track.
What is the full path name of that track, according to MC? (You can examine the Filename field in a column, or use the tooltip).
What is the full path name of that track, according to Windows Explorer?
Probably they are different paths, and you need to tell MC about the new location. This can be done with the Move, Copy & Rename tool, were you can replace invalid portions of the path with the correct path (use the update library mode, not a Move mode). Ask if you need more help.