A couple factors in play here.

- If you request a mileage upgrade, it must clear in advance. It will not go to the gate because gate agents don't have the time to verify balances in your account, pull miles, reissue the ticket, etc. (You may convince a friendly and knowledgeable gate agent to do this anyway, but this is rare.)

- JFK-LAX (and JFK-SFO) used to never open up inventory in advance due to the premium service on the route. All upgrades were done at the gate. There is some evidence of this loosening up, but advance upgrade availability still remains quite tight.

So on that route, upgrade inventory is rarely released in advance... and if your mileage upgrade doesn't clear in advance, it won't clear at all.

Moral of the story: don't upfare unless you can lock in the upgrade. At least you only required K or higher. Some people learn this lesson on international M fares.