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This is normal.

iOS puts significant limitations on what most applications can do in the background. In general, OmniFocus can only do stuff (like start a sync) if you bring it to the foreground, or if some other event in the system wakes it up (a timer or a location event).

One idea we've got in our "Someday/Maybe" queue is to be able to set locations where OmniFocus should try to wake up & sync, as Instapaper does.