New email not appearing

[rob+mutt]

I'm quite new to mutt, and so far very happy with how it works, and how
configurable it is.  I've got a problem that is probably just me missing
something obvious, and would appreciate a pointer.  If mutt is running, and I
receive new email, mutt doesn't add the message to the index list until I hit
a key.  After I hit up or down, or ctrl-L, the new message shows up.  Hints?

TIA,
Rob