Over half of my music library is not showing when plugged in?

[AC Question]

I just transferred to a 64GB SD card on my phone from a 32GB. I moved all my music over a few days ago, and I went to add a new song today but over half of my library is not showing up on my computer. I can see and play the songs on my phone, but they refuse to exist on my PC. Why is this happening and what can I do to fix it?