Animations and issues of rigging from Mixamo to Unity

I'm on a group being with the task to create and animate a character on unity.

I found Mixamo and worked quite well on adding the skeleton and animations but with some breaks on the bones. The arms and the head for example. I tried to t pose the skeleton but nothing at all.

Is there a way to change the weights of each bone on Unity or Mixamo?.

https://gyazo.com/a1b71d270565d8910fe33198056d170b