On AA flights, it's standard for FAs to announce that most roll-aboards fit in the bins wheels first and that passengers should stow them that way. Whether or not said passengers listen, and if they listen whether or not they comply, are separate issues. Given congestion in the aisles, all the other stuff that's likely to have been shoved in around a longitudinally-stowed roll-aboard before an FA passes it, how long it would take to remove everything to reposition the offending piece of luggage, and the passengers whose boarding would be held up while that was going on - the cabin crew's ability to fix things is in practice limited.