Can sync some local mp3s, but not all

[earlz]

So, I have some local mp3s on my desktop that I added to a playlist and synced to my HTC One M8. My problem is that some of these MP3s synced, and some didn't and just say pending. I've confirmed with mutliple different programs that these MP3s are valid and playable. Why are they not syncing? 

[Daniel]

Do they play OK if you manually move them to your phone? The container may be .mp3, however they may be encoded using a format not playable by your actual phone.