Expression involving square roots not simplifying

 expr = (Sqrt[s γ^3] + 2 Sqrt[s γ^7] + Sqrt[s^5 γ^7] + Sqrt[s γ^11] + 
        2 s (Sqrt[s γ^5] + Sqrt[s γ^9]))/(γ (1 + s γ + γ^2)^2);


Simplify[PowerExpand[expr]]

Mathematica graphics


More by way of explanation of the "indifference" that causes Simplify to not budge. In order to factor the expression so that it can be reduced, all the square-roots have to be factored and initially the complexity (computed by Simplify`SimplifyCount, which is equivalent to LeafCount on these examples) remains the same:

Simplify`SimplifyCount[Sqrt[s0^5 γ^7]]
Simplify`SimplifyCount[s0^(5/2) γ^(7/2)]
(*
  11
  11
*)

The actual algorithm used by Simplify is unknown (to me), but it makes sense to reject a transformation that results in an expression with the same complexity as measured by the ComplexityFunction (to avoid getting stuck in an infinite cycle of equivalent-complexity expressions).

While there is a simpler solution (see @Nasser's), another approach is to tweak ComplexityFunction to make the desired steps seem "simpler":

cf = LeafCount[#] + 2 Count[#, Power[_Times, _], {0, ∞}] &; 
Simplify[(Sqrt[s0 γ^3] + 2 Sqrt[s0 γ^7] +
   Sqrt[s0^5 γ^7] + Sqrt[s0 γ^11] + 
    2 s0 (Sqrt[s0 γ^5] + Sqrt[s0 γ^9]))/(γ (1 + s0 γ + γ^2)^2),
 γ > 0 && s0 > 0, ComplexityFunction -> cf]

(*  Sqrt[s0 γ]  *)

Raise the coefficient of Count[] in cf to 5 and the result will be Sqrt[s0] Sqrt[γ].