JFK and MIA are both AA hubs, so it's probably just because they're "repositioning" a longhaul plane with lie-flat seats that came into JFK from far away but is next going to be used from MIA for the next far away flight. Or else, just taking up spare time between longhaul flights to/from JFK or to/from MIA with that plane. (If a plane flies across the Atlantic and then uses up somewhat over half a 24-hour day, rather than wasting that plane for the rest of that 24-hours, they may flight to some other hub and back before flying it back across the Atlantic.)

AA tends to fly lie-flats on non-transcons domestically only between its various hubs. And of course AA doesn't have any hubs in the Carribean (nor do any other airlines).

Perhaps you should consider a connecting flight through MIA, and be happy that you can at least get lie-flat part of the way, as you're not that likely to find lie-flat going all the way to the Caribbean.

At AA (and probably most other airlines), lie-flat tends to be only on planes configured for JFK-LAX/SFO transcons plus planes configured for longhaul international travel. To get a lie-flat on a shorter flight (which is all you seem to want), it has be a transcon plane, or an international plane "using up" time domestically.

Keep in mind that the primary reason AA puts lie-flat on planes is for business people booking business class and paying business class fares so as to get the best use out of their flight time, not for the casual traveler going on vacation. And there's comparatively little business travel to the Caribbean.

Thanks!

Do you consider LA warm? You can get lie-flats from JFK to LAX, but they are a bit pricey. If you're willing to schlep or take a short train ride or flight to PHL, AA flies a 330 from PHL to LAX and it is usually a decent deal.likewise, you can fly or train up to BOS and catch the JetBlue Mint seats to LAX for usually a better deal.