Scapula extension

I have an issue with fully extending my upper back while practicing planche positioning. In doing planche leans and scapula pushups my mid rounds and extends upwards more than my upper back. This gives me an upsidedown v-shape look rather than a rounded back shield look. I know to push as though I am trying to touch my front delts together. I am not sure if this is a felixbility issue, a mind body connection issue, form or strength. Thanks for an advice. I can post pictures if it helps.

The upside down V shape is very common in the beginning. Often the solution is just a matter of tucking the hips while simultaneously lowering them.