Neck bands remain after surgery
I'm sorry you are unhappy with the outcome of your surgery. The visible band in the midline is where your surgeon sewed the two muscles together. This can form an unsightly band in thin patients. It might improve with time and I would certainly give it 3-6 months before making a final judgment. A little Botox might also improve this.
The surgical fix would likely involve releasing the midline sutures and lifting the muscles from an incision around the ears; essentially a full face lift.
Seek a second opinion from a few surgeons experienced in revision neck and facelifts.
Best of luck.
Neck bands after platysmaplasty
sorry you're having issues. This can sometimes just happen from swelling or contracture of the bands. More often it's because of excess skin laxity or because not enough skin was released and it was pulled midline with the muscle being pulled midline. If skin laxity is not the issue, the easiest thing is to repeat the procedure and release skin as far lateral and interior as possible. If there is skin excess then you have to consider removing some midline (which I don't recommend for you), performing a vertical neck lift from around the ears which would definitely fix it but is a more involved procedure, wait it out for a while with injections, or heating therapy like profound which would only give a slow and modest improvement