Assuming front passenger seat -

If you hear a buzzing in the seat when you press the headrest switch then remove the back panel of the seat and check that the headrest post is engaged in the plastic riser. Most likely the plastic riser is cracked. It's a $15-20 item at the dealer.

Another possibility is a damaged or broken cable between the motor and the riser. I think the springiness of the cable keeps it place so it's just a matter of pulling off the old one and fitting a new one.

To remove the back panel, remove the 2 Philips head screws that go up into the back panel along the bottom edge. Pull down the back panel a couple of inches then remove it. It should be obvious how the headrest mechanism works.

If there is no buzzing when you move the switch, spray contact cleaner into the switch to see if that gets things going.

Sixto
... stay tuned