Most loops won't preview or auto preview - that about says it... sending the output to the master bus with a limiter so I know it's not peaking out... doesn't matter if it's an mp3 or wav... only 1 out of every 5 will preview.

If the correct preview bus is chosen, the only thing I can think of is that you're previewing in loop mode and you select a new loop before the other one has finished. So my theory is regardless of whether you hear the old loop playing or not, the newly selected loop won't play until the old one is finished. 
 
Try turning off loop mode to see if you can preview files as soon as you click on them.

I was involved in a similar thread a while back and developed this formula for failure and the steps to avoid it http://forum.cakewalk.com/FindPost/3255769
 
Also make sure "Preview at Host Tempo" is disabled for regular waves and mp3s. But the skipping entire files sounds like the post I linked to above.

thanks... i had loop mode off... I had no bus assigned as the preview bus until this incident, but then I forgot to do the FIRST thing that should be done anytime something doesn't work and you change settings... RESTART the program!  It works now.