Missing a lot of emails from BES

I have a BB 8520 Curve which is under BES. The BES server should forward every email from exchange server to my BB. But the thing doesn't work like that way. It always randomly forward some email to the BB and skip others. I can't find any rules what emails are been forwards or not.

PS. I disable wireless sync'ing contact and calendar, which might not be the cause of the issue.

Any ideas? Thanks.

Under 'services status', I can see BES is connecting through Wifi. But in Wifi diagnostics, in Enterprise section, the connecting/Authenticating router/Authenticating server all show a grayed slash. Does this have anything to do with connecting to BES through Wifi? Would this be the cause of my missing mail issue? Thanks.

BTW, could the BB connecting to BES through any personal Wifi? Is this default behavior?