For full details, here's the FAQ article:
Some email comes to my BB and some does not - BlackBerryFAQ.
It could be sporadic and not total depending on the frequency with which your email client is set to check for email. In other words, if it downloads new email very frequently, then fewer, or none of the messages would be grabbed by the BIS server. If set to check very infrequently, then most messages might be grabbed by the BIS, and some not.