OWA & delegate access

Exchange 2003 on Windows 2003 with SSL certificate...

User A has given User B delegate access (read and write) to his calendar.
User B can use OWA (IE 6 and
and User B's username+password to view User A's calendar. But User B
have a button in OWA to add a new item on User A's calendar.

What am I missing so that User B can add items to User A's calendar with

I'm sorry if this is obvious, but I'm apparently having trouble coming up
with the
right combination of terms for a Google search to help me.