You're all too familiar with the symptoms. Year-to-year volume is dropping, not rising like it used to. Your ticket average is stagnant. Customers either aren't buying extra services, or they're buying only the low-dollar extras. So your greeters aren't very motivated to earn commissions.
It's a cycle you can't seem to break. Sales are down, therefore commissions are down, and consequently your greeters are not performing. What do you do?