PSP Video 9 MKV set backs

I am using PSP Video 9 to get shows and videos to play on my PSP, and almost everything works fine. ALMOST.when it comes to MKV files the subs do not show up once it has been ran through the PSP Video 9. any ideas on why and how I can fix it?

use pspvc (psp video compressor)-http://pspvc.nswardh.com/