Dynamic Workstation group filter criteria for model

I want workstations to add to Dynamic Workstation Groups based on model of computer, e.g. Dell Vostro 3560. That is exactly how it appears in inventory in Zen 11. However, if I use Filter Criteria of "Inventory - System Model" it does nothing. What am I doing wrong?

Also, I can't find a detailed explanation of these filter criteria in any Zen 11 Documentation. Can someone please give me a link to it?