Why can't I see my files?

[AC Question]

Over half of my music is not showing up from my SD card when I plug my phone in for file transfer. It's on there, I can play it on my phone, but it won't show up when plugged in... Why not?

 

[ManiacJoe]

Using a file browser app on your phone, are your music files located where you think they are?
Some music apps will display the collection of music files in one large list regardless of the many folders involved.

 

[Forumpy]

Yeah, I can see them on ES file explorer. It's just when it's plugged into a computer that I can't see them.

 

[ManiacJoe]

Mac or Windows? What version?
Which phone?
Which version of android?

Are the unseen files all in the same folder or scattered all about?
Internal memory or SD card?

 

[Forumpy]

Windows 8
Oneplus X
Android 6

All the folders/files are all in the "music" folder. Within there there are obviously multiple folders and loose files, and half of those folders and all of those loose files don't appear.
SD card. I recently upgraded to a 64GB and that's when the problem started. I've never had this issue before now.

 

[Forumpy]

Any ideas? Literally no other place I've posted this question to have even responded.