joints wont break when a surface changes to smooth?

while true do

wait()

local hitbox = script.Parent:FindFirstChild("GenHitBox")

if hitbox ~= nil and hitbox.TopSurface == "Smooth" then

script.Parent.GenHitBox:BreakJoints()

end

if hitbox == nil then

return

end

end

Im trying to break a parts joints when its top surface is changed to smooth. no errors come up but the joints will not break when the top surface is changed to smooth.