Simplify does not work in V12.0! please help

I am trying to simplify this so that the power of sigma in the denoimator gets simplified. Please help!

Simplify[!(
*UnderoverscriptBox[([Sum]), (k = 0), (n)](k 
*SuperscriptBox[([Sigma]), (1 - k)] Conjugate[
    a[k]]))/[Sigma]^2]

The output is

!(
*UnderoverscriptBox[([Sum]), (k = 0), (n)](k 
*SuperscriptBox[([Sigma]), (1 - k)] Conjugate[
   a[k]]))/[Sigma]^2

Interestingly, when summation sign is removed, it does simplify without ‘Simplify’!