I've had this same problem back over the summer with iOS 7 beta. I'm not sure if it's an iTunes issue or an iOS issue because it happens sporadically and actually just happened last week as well. Try unplugging your device and perform a hard reset on the device, them sync those songs with the little perforated circles next to them over again. If not, you'll have to clear out all the music on your device to recache the library cache on your computer, then sync your songs over from scratch again
Also install the latest iTunes update. I haven't myself but it may have resolved the issue
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