Technically, it's a violation of FAA regulations, if the FA was not visible in all cabins. In my 50+ years of flying, I have only seen them miss the demo twice.

Don't all of AA's 738 have more than 150 seats now? That required 4 FA's on the flight. There should be NO walking from F to Y on the 738's. There should be 3 FA's in the aisle during the demo: one F, on forward Y, one at overwing exits in Y.