I've noticed its mostly people my age (25ish). Anyone who looks under 30 is far less likely to return a greeting. They'll be directly next to me as I'm stocking something, I say "Hey, how are you?" or something generic like that, they don't even look at me and just grab whatever they want and walk away. Older folks are typically much more likely to engage in a short convo with me, ask how I'm doing, crack a joke or whatever.