Required to check bag in group A?

I brought my usual rollerboard which fits comfortably in the “efficient” way (wheels to the back) on the A321, and on every JetBlue flight in the past. Gate attendant insists I need to check it for free. She says it must’ve expanded since I brought it on my flight on the way out 😆. I get on the plane and there are many bags larger than mine overhead. What gives?