Disklavier

As a new owner of XG Series, and a new DUG member, I am having 
difficulty with downloading & using Midi files.  Some files will play 
just fine, while others the unit reads "Cannot execute". From my 
point of view, both files "look" the same.  

Tried downloading some of the wonderful music from Derek, on this 
site & could listen to it on my computer, but unable to get the 
Disklavier to cooperate.   I would appreciate any feedback as to what 
I may be doing wrong.  Love the unit, but find much of the commercial 
software available not to our liking.  We love a soft gentle 
background type music, and not finding much from Yamaha.

Thanks again for helping a newbie.

Dick