One of the features that came with OS 4.5+ was the ability to have multiple calendars. BES users are used to only having the one BES calendar. Now there is one for each BIS email address along with another one for Facebook if you have it installed. The issue is, sometimes a meeting request will be sent to a BIS account instead of the BES account and then they don’t sync.
RIM has provided some instructions in this knowledgebase article on how to get around this multi calendar issue by permanently collapsing all of the calendars into one calendar.
NOTE: These instructions will permanently move the calendar entries from one calendar to the default calendar. There is no way to undo it.