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.