Which version of the game are you using and what mode are you playing in? Fictional, historical, modern-day MLB rosters? Are you using the default MLB quickstart or someone else's roster set? Which version of OOTP are you using?
Assuming that you are using the included MLB rosters in OOTP 8 pre-version 13, there was a fix for this issue provided that you re-download the game:
Again: OOTP 8 patch released! New version # is 8.0.0.13
Does that solve your problem?