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It sounds like these songs are having issues downloading from iTunes in the Cloud. To troubleshoot, do the following:
Try signing out of your account in Settings > iTunes & App Store. Once signed out, go ahead and restart your iPhone, then sign back in.
If you continue to have issues, there may be an interrupted download that is preventing other content from downloading. See the following article to resume any interrupted downloads:
If the music has been purchased from the iTunes store, you may be able to download the music directly to your device:
Also, this process can be set automated in order to automatically download any future purchases to your iPhone.
I hope this information helps ....