Problem in DSP

I'm working with DSPF2812 and i have a problem . i'm doing PWM by comparing sinewave have 50 Hz and sowtooth 10 Khz but I can't use greater than 2 KHz for sawtooth where when i used greater than it the output square pulse didn't have 50 Hz..... why?

i wuld like to tell you i'm using matlab2006a simulink to perform block of PWM and code composer V3.1.

thanks alot