Some but not all music is showing in my phone's media player I have a Galaxy S7 Edge, and am using the 64GB SD card they gave with the phone when it first came out at BestBuy's. I've put music on my phone already and have had it stored on the SD card and have never had an issue changing up what I have on there until yesterday. I re did my playlist and put different music on and synced my phone only to get to work and find out certain songs do not show up in the default media player (even though they are songs I've had on my SD card before). So today I decide to re sync it and after it is done I went through Windows Media Player and it said all the music was "Already On Device". Go to the default media player on my phone to a specific artist, and only HALF of their albums are on there... And when I say half I mean, it basically skipped every other song on every album. So half the album is on and the other is not... But when I search files on my phone all the music shows up.

I know there were other people with similar issues that I saw but none were resolved, and none were having issues only with partial music not showing up. All my artists show up, but it's only putting HALF the songs they have from each album into the 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.