Datagrid colmun width * question

hi

i have a WPF app , in my main view i have a stack panel with 2 group boxes , each group box have a datagrid inside 

it look like this

when i try to change the width of the last column  "notesColumn2" to be * instead of a fixed size , so it take all the space available , this what happened

all the columns have wrong width , why ?

note if the grid where outside the stack panel , the * with work fine