I have an issue in the following equation. I just can't figure out why the equation (-a^2 + a^2y - yb^2) is not getting simplified from T?

    In[1]:= T=(s^3*b(-a+2*
            a*y+4*b*y^2-2*b*y))/(Sqrt[s^2*b^2*(-4*
                y^2*a^2+4*y^3*a^2-a^2+a^2*y-4*y^3*b^2+4*y^2*
                b^2-y*b^2)/(-a^2+a^2*y-y*b^2)]*Sqrt[-s^2*(-a^2+a^2*y-y*b^2)])


    Out[1]= \!\(\((b\ s\^3\ \((\(-a\) + 2\ a\ 
              y - 2\ b\ y + 4\ b\ y\^2)\))\)/\((\@\(\(-
                s\^2\)\ \((\(-a\^2\) + a\^2\ y - b\^2\ y)\)\)\ ?\((\(b\^2\ 
          s\^2\ \((\(-a\^2\) + a\^2\ y - b\^2\ y - 4\ a\^2\ y\^2 + 4\ b\^2\ y\^2 \
    + 4\ a\^2\ y\^3 - 4\ b\^2\ y\^3)\)\)\/\(\(-
                a\^2\) + a\^2\ y - b\^2\ y\))\))\)\)

In[2]:= Assuming[a>0,Assuming[  b>0 ,Assuming[ s>0 ,Assuming[  y>0,Simplify[T]]]]]

Out[2]= \!\(\(s\ \((\(-1\) + 2\ y)\)\ \((a + 2\ b\ y)\)\)\/\(\@\(\(-a\^2\)\ \((\(-1\) \
+ y)\) + b\^2\ y\)\ \@\(\(b\^2\ \((1 - 2\ y)\)\^2\ y - a\^2\ \((\(-1\) + y - \
4\ y\^2 + 4\ y\^3)\)\)\/\(\(-a\^2\)\ \((\(-1\) + y)\) + b\^2\ y\)\)\)\)

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