COT320 isn't a good paper for pure platinum prints. The gelatin sizing binds to the potassium chloroplatinite and hinders the chain of reactions which forms the image. Try printing on the reverse side where there's no gelatin. As far as I'm aware there's only one paper which works for a pure platinum image 'out of the box' and that's Buxton.
What would I have to do to other papers to make them ready? Oxalic Acid pre-bath? or something else?