What is wrong with "Tu as dû montrer une autre attitude"
I see nothing wrong with you sentence :/
Correct. You can say 'tu as dû montrer' or 'vous avez dû montrer'.
Why is "vous avez du montrer une autre attitude" wrong please excuse lack of circumflex
There is nothing wrong with it. Please report it when you get the chance.
What about: "Tu devais presenter une autre attitude"
Vous deviez have never been shown in the options. twice in a row...