Some but not all music is showing in my phone's media player
This may not be your issue, but I've had a problem like this for years, and just figured it out today. I wanted to get it written somewhere.

I buy CDs (yes, I'm the one), and then rip them to .ogg files for playback from my phone. What I was finding was that the newer CD's often wouldn't show up at all on any Android media player. However, the files were definitely there. If I connected to my PC with a USB cable I could play them just fine on the PC.

Hat tip to Metrov in this AndroidCentral thread. The problem appears to be that Android media players are very persnickety about music files having their tag information properly filled out. I installed Florian Heidenreich's Mp3tag, ran it against the problem children, and sure enough, they didn't have their tag info filled out properly. I'm guessing its been more of a problem for me with new music, as newer stuff is less likely to have good tag info about it uploaded to freedb.

This also allowed me to fix another problem I had where one album was showing up twice (inconsistent artist and Album Artist) tags.

In your particular case, I'd suggest hooking your phone up to a computer with a USB cable, then running Mp3tag or a similar tag editor and comparing the tag info in the files that are showing up on Android to the ones that aren't.