Hello ,
Welcome to the Apple Support Communities!
From the way you describe your situation, it sounds like the some of your purchased music may not actually be located on your iPhone. If, on your iPhone, under Settings > Music you have Show All Music turned on, you will not only see what is on your iPhone, but all other purchases that are not currently on your iPhone. This music that is not on your iPhone is available for download from the cloud.
Please review the attached article for more information.
Download past purchases from iTunes in the Cloud - Apple Support
On an iPhone, iPad, or iPod touch
- Tap Settings > Music and make sure Show All Music is on.
- Tap the Music app.
- Find the item that you want to download.
- Tap the Download
icon.
If you can't find an item in Music using these steps, download it from the iTunes Store app:
iPhone or iPod touch: Tap the iTunes Store app. Then go to More > Purchased. Tap Not on This iPhone or Not on This iPad. Scroll to find the song you want to download, then tap it. Tap the Download
icon next to it. Your item downloads to your device.
Have a great day,
Joe