Transporter/Squeezebox won't play some tracks

I have a Transporter and a Squeezebox Classic and I am using iPeng 9 and LMS 7.9.0. For some reason, certain tracks and even entire albums won't play. These are generally albums that I have ripped either with iTunes or Max on a Macintosh. The tracks play fine in iTunes, just not iPeng or the Squeezebox app, both on an iPad and iPhone. Any reason why this might happen?