Hmmm...
There's a couple situations that might be relevant described at the bottom of this page:
http://www.omnigroup.com/products/om.../support/#siri
If that doesn't get this sorted out, please contact our Support Ninjas for more thorough investigation:
omnifocus@omnigroup.com or 1 (800) 315-6664 or 1 (206) 523-4152