Generally speaking, if MC won't import a file it means one of two things:
1. The file extension is not recognized by MC as being part of the import type you are trying to do. For example, if you are trying to import an MP4 file (which is usually video) and you only have audio checked, MC will skip the MP4 file(s). Or if you're using some unusual extension that MC doesn't recognize, it will skip it.
2. If you have previously imported a file and then deleted it from MC, MC keeps a record of these deleted files and it will not import them again. You can work around this if this is the case.
Brian.