Arches platine as COt 320 need to brush a little longer compare to other paper. If not some of chemical will stay on the paper ( not in) and will wash in the developer. You can use as suggest by Dave some twenn 5 you can dilute @1%) and just add a drop to your mix. Too much tween and solution wil get deep in the paper.
It is a quite commmon problem with Arches platine