How to do this in an older version of Outlook?

I’m using Outlook 2003. When composing a new email, I have it set so it will suggest the recipient when I start typing. I used to be able to press ‘enter’ and the selected address would appear in the address box. But recently it has stopped working: it still suggests, but when I press ‘enter’, the cursor changes to the little round ‘loading’ thingy, goes round and round for a bit, then changes back to the cursor and nothing has been entered into the address box. Does anybody know how to outsmart this, and make it start working again?