Blue sketch articles and problems go hand in hand.
You are using the Move (face) Tool, when you can just click drag the blue line and the extrude will update for you.
This way the sketch will always be the same as the body.
You have 4 extruded (new bodies - not joined?) bodies before the Mirror, all 1,2,3 are the same thickness 4 is a different thickness.
The Mirror works fine. Maybe not selecting all bodies just one, is the problem.

A hole here is not good.
Use Create > Project tools when sketching to copy parts of an early sketch to the current sketch,
Fusion is expecting better accuracy than this.
Black and purple lines are better.
Might help...