Figure out why they are resistant and find some means to reduce their resistance. For example: Did they write the code? If so, they may feel you are 'attacking' them/their competence in some way and you would need to find a means to reassure them that you respect them (yadda yadda) while still arguing for improvements. In many cases, ego and other personal stuff gets in the way of productivity. When that happens, the most effective thing to do is address the two issues separately.