how do i make my variable update?

I try to make a bag which u can pickup but the variable does not update so when the bag moves you cannot pick it up anymore becouse the variable is stuck on 1 coordinate point anyone help with the Mag variable?

inputservice.InputBegan:connect(function(i,g)
	if i.UserInputType == Enum.UserInputType.Keyboard then
		if i.KeyCode == Enum.KeyCode.F then
			for _,Door in pairs(workspace.Shops:GetChildren()) do
				local Mag = (Door.Center.Position-game.Players.LocalPlayer.Character.HumanoidRootPart.Position).magnitude
				if Mag <= Door.Range.Value then
					Door.Event:FireServer()
					break
				end
			end
		end
	end
end)