Why aren't my songs/podcasts "able to play" on my Lumia 735?

[Windows Central Question]

I'm transferring .mp3's straight from my computer using the instructions here http://www.windowsphone.com/en-us/how-to/wp8/music-and-videos/sync-music-ringtones-and-podcasts-using-my-computer. Why does it say they are "unable to play this file."? They are .mp3's with .mp3 extension. I will add that they are downloaded podcasts that are nearly 3 hours long each, could this be part of the issue? Thanks for your help and time.