Why aren't certain songs playing in nike running app?

I have playlists in my iTunes that I listen to when I run, but ever since I got the iPhone 6 plus, more than half of the songs don't show up when I use the Nike Running app. The songs play fine when I'm playing them through iTunes and I made sure to sync the songs to my phone from my computer, but for some reason they won't play in the Nike app. This has been a problem for over 6 months now. I waited to see if software upgrades and/or app upgrades would solve it, but it hasn't. Everything is updated.

It's pretty annoying to navigate back and forth between iTunes and Nike while running just to hear what I want to hear.

Any help is greatly appreciated. Thanks