Running Outlook for Mac 2011 - v.14.4.9 on a MacBook running Yosemite v.10.10.3

Occasionally I find that messages in my InBox disappear. They've been in that box for a few hours or a few days, and they just disappear. I can't find them elsewhere. Rules don't seem to be involved. Other messages remain there.

The accounts that feed into my mail are imap accounts.

What can I do to stop this disappearing act? Do I have to create a new identity? If so, what do I do with the half dozen older ones I've accumulated by now?

Thanks in advance,

Wendy