Had that same thing exactly happen a few times, finally gave up too. Most tills today don't contain change for a 20 anyhow.
A "twenty" seems to have become the most common denomination, with ATMs issuing no smaller bills. Getting ready for a gun show means a trip to the bank, because everyone who buys a $2 used ammo box or a $5 scope base whips out a twenty on you.
I like young people, but make it a point to find a register with an old person. It's just easier. Sometimes, I'll get "cash back" so I have lunch money for the week - usually a "twenty." The older checkers always ask if you want that broken down. Once I said, "yeah, a ten, a five and ten ones."
This yielded a wry smirk from the mid-sixties cashier and she said "nice try." The response was reflexive and quick - she did not have to think about it for even a second.
I did encounter a sharp kid the other day, going through a drive-through (which I hate doing) when I bypassed the first window and she leaned out the window and yelled "BACK HERE!" They NEVER have that window open. I backed up and paid her and told her I didn't expect to get my food for free. She smiled and said "wrong day anyway - we only have free food on the second Tuesday of each week." I had to tell that girl I was proud of her for being so quick with a corny old-people joke.