GetMouseDelta requires the MouseBehaviour to be LockCenter or LockCurrentPosition to work. How are you getting the Delta? You haven’t provided your method.
If you would like scroll wheel delta you need to use UIS.InputChanged and check if the InputObject’s UserInputType is MouseWheel, then you can get the “delta” from InputObject.Position.Z.
UserInputService.InputChanged:Connect(function(input)
if input.UserInputType == Enum.UserInputType.MouseWheel then
print(input.Position.Z)
end
end)