Tell you what: stop looking at the customers as walking wallets, and you'll have your respect. Greater tips too. Guaranteed. 99 % of the time, it's the staff's fault.
And repeat this every morning: When the customer receives astonishing service and a mediocre product, it’s likely they give you a tip, maybe try again. If it’s the other way around, unlikely, and the enterprise is just a fluke. Do I expect undergrads working for tips to know this? Yes, god dammit!