You can, actually, have BES and BIS on the same device (I do it, as do many others here). However, it is possible for the BES to restrict being able to send AS the BIS account.
What happens when you try to send as the BES account? Do you get a red X? If so, open the message and scroll up - what is the Message Status?
No longer a BES Admin, but it was fun while it lasted!