As I mentioned in other posts, my son is a manager, trainer and server at a national chain here in Canada.
MY GF and I were in his restaurant last night and 6 young men (perhaps late teens to early twenties) were thoroughly enjoying the all-you-can-eat diner at one of his tables.
We were watching and listening to my son very carefully and he was friendly, provided prompt service, etc.
Their bill was about $150 and the tip: $6.
My GF's theory is that maybe these young men just do not know what a 'reasonable' gratuity should be, assuming that you are satisfied with the service.
She is suggesting that maybe the chain should implement some sort of polite and obviously well thought-out suggestion in a predominant location on the menu.