Check your BBdesktop program sync settings. I don't have that in front of me but I am sure there is something in the sync settings on how far back to keep appointments.
I'm doing this from memory, as I don't have my blackberry with me right now.
Plug in your blackberry, open up desktop manager, go to synchronize, click on the configuration tab at the top, click on configure sync, click on calender.
In the bottom right hand corner there should be a button that says configure I believe, then click on advanced settings, there should be an option that says "synchronize calender events", set it to all calender events, as apposed to future events only.