Obtuse neck after deep plane lift - how can I achieve a more defined neck? (Photos)

You have redundant midline skin with minimal subcutaneous fat. The skin excess is hard to eliminate without including the incisions behind the ears/neckline as this is how we pull out the extra skin and eliminate it. Be very careful with revision surgery in the neck, especially with very thin skin!

The five-month photo shows the presence of the medial borders of the platysma. At such a young age, this appears to be due to the platysma as opposed to excess skin. We don't know what type of platysmaplasty was done, but based on the appearance, the obtuseness appears due to underlying structure with either recurrent or incompletely released platysma.
The MyEllevate procedure can release the medial border (cut across it so the continuity is broken at the cervicomental angle) and improve the CM angle in a case like this. It is a non-incision permanent suture-suspension necklift, done with specifically designed-instrumentation under local tumescent anesthesia. The suture is used to cut the medial border of the platysma. The suspension suture is then introduced through tiny punctures that do not leave a scar.
Although pulling the skin laterally using the incision behind the ear could be tried, if the problem is banding of the platysma, that may be ineffective or temporary.

It looks that platysma muscle bands visible underneath the thin skin are the problem rather than extra skin? Correction of these using sub-mental incision would be my approach to the problem. Possibly some skin tightening by means of radio frequency or J plasma?

Thank you for posting your question here!My answer will be based on the limited data and pictures provided , for a more detailed answer/plan please consult with your plastic surgeon in person.
It is hard to tell for sure what went wrong without knowing what was done exactly , I'm seeing a couple of bands under your chin that I am guessing is bothering you. They seem to be the result of skin redundancy that might have occurred due to a tight platysmaplasty in the midline (the muscle was brought together in the midline (this is a standard part of the neck lift ) but might have been brought too tight) and some of the recent teachings in neck lift advocates not dissecting the majority the skin away from the muscle due to many perceived benefits but that sometimes can result in bunching if the muscles are brought together too tight in the midline. That being said , this is all an assumption , I would recommend discussing with your surgeon so that they can come up with a plan that works for you. I hope this helps!, best of luck !