Of course it would be the letter from the professor currently on sabbatical that would be missing - but I was lucky enough to catch her at her email and get her to re-send a copy, which subsequently arrived successfully. For the GRE scores, the WWU graduate school was kind enough to cc me the letter they sent, so all I have to do is fax it over (or make a copy and send it to them by post).
At least it makes me feel justified in my mild obsession with getting things in early and regularly checking on my status.