Tom, if you open up OmniFocus' Settings screen and scroll down to the bottom, what version is installed on your phone? If you have version 1.8, it's possible that
a bug in that version is causing this.
If so, you'll be able to make the issue go away by updating to the most recent release. (Version 1.8.2, as of this writing.)
If you're using that version, give our support ninjas a call at 800.315.6664 or
send us email and we're happy to investigate this with you!