Hello Shelby3698,
Thanks for choosing the Apple Support Communities. We understand that Automatic Downloads for music added to your library turns off unexpectedly. We'd be happy to help take a closer look.
Let's start by checking how much available storage is on your iPhone and how much space currently downloaded music is using following the steps here:
How to check the storage on your iPhone, iPad, and iPod touch
If you have an Apple Music subscription, we'll also want to ensure that Sync Library is enabled as mentioned here:
Add music to iPhone and listen offline
Next, go to Settings > Music and turn on Automatic Downloads again. If Optimize Storage is enabled right above that setting, check that the minimum storage selected exceeds the number of songs currently downloaded to the iPhone, leaving room for new songs to be added. Let us know what you find.
Cheers!