Hi Forestfairy49,
welcome to the Apple (user-to-user) Community.
Re: "music sometimes will play from the Music App and sometime not"
Wondering if you have a subscription to Apple Music, or not.
Without a subscription, we can still use Apple Music, yet not all features.
Subscribe to Apple Music on iPhone
Eg: Apple Support's article (link below)
Subscribe to Apple Music on iPhone – Apple Support (AU)
Says:
“If you’re not a subscriber, you can use Music to listen to music synced to iPhone from a Mac or a Windows PC, play and download previous iTunes Store purchases, and listen to Apple Music radio for free”.
Previous purchases may be available in the "Library" in the Music app.
or
If needed:
How to: Redownload music - Apple Support
With or without a subscription:
How to: Listen to music and more in the Apple Music app - Apple Support
If needed:
The Music Support Website has info.
There's a "get Support" link at the bottom of the webpage, link below:
Music - Official Apple Support
All the best :-)