some how lost half of my music on iphone how can I get it back?

The following steps should resolve things assuming all of the content you want is in your library.

(If it isn't see Recover your iTunes library from your iPod or iOS device.)

1. Backup device.
2. Restore as a new device.
3. Restore the backup you made earlier.
4. Review the sync setting and for Music choose sync with selected playlists, then select some.

N.b. A feature introduced with iOS 9 referred to as app thinning or app slicing allows each device to download only the code and resources needed by that device, resulting in smaller downloads and better use of storage on the device. As a consequence the device no longer holds the universal version of the app that could be installed on any device so transferring apps is no longer supported. You can however download past purchases from iTunes and/or enable automatic downloads of app purchases to make sure that iTunes always has the apps that you want on your device. A further change to the way restores are implemented means that all private app data is restored and apps that are not in the library are queued for download from the iTunes Store over Wi-Fi, in a similar fashion to the way restore from iCloud works. Not having the apps in your library isn't the drawback it once was, however you are still at risk of losing a favoured app if it is removed from the store and you don't have a local copy.

tt2

Hello there, BigMexican68.

It sounds like you're having issues trying to sync purchased content from iTunes to your iPhone 6. The following Knowledge Base article offers up some great information in regards to how to accomplish that:

Move content from iTunes on your computer to your iPhone, iPad, or iPod touch

By default, iTunes manages the content of your iTunes library (including your music, TV shows, movies, and more) manually, which gives you control over the content you want to take with you on your iPhone, iPad, or iPod touch. This is useful if your iTunes library contains more items than can fit on your device.

If you're a member of Apple Music or have an iTunes Match subscription, you have iCloud Music Library turned on. When you have iCloud Music Library turned on, you can't manually manage music. You can still manage videos using the steps below.

Manually add items to your device

1. Connect your device to your computer.
2. Open iTunes. Make sure that you have the latest version.
3. Choose the content that you want to add to your device from your iTunes library.
4. Drag the content to your iPhone, iPad, or iPod. A panel should appear. Locate your device.

If you're using an iPhone, you can only add music or video from a single iTunes Library.

If you're using an iPad or iPod, you can add music from multiple iTunes Libraries, but video from only one iTunes Library.

If you can't manually add or remove content from your device

Check to make sure that the "Manually manage music and videos" option is turned on:

1. Connect your device to your computer.
2. Open iTunes. Make sure that you have the latest version.
3. Locate and click on your device.
   
4. In the left sidebar, click Summary.
5. Scroll down to the Options section.
6. Click the box next to "Manually manage music and videos."
   
7. Click Apply.

You might see a message that your device is synced with another library. This means that you've used a different computer to sync your device in the past, and iTunes can't sync without first erasing and replacing all iTunes content on your device.

Thanks for reaching out to Apple Support Communities.

Cheers.