I think something's terribly wrong with that paper (sizing?)...

Vandyke's not that much picky with paper but being an iron-silver process, it requires a neutral or acidic surface for best results - you can't get the best out of it with buffered papers. (Which Arches paper is that? Arches has many different papers...)

Can you please describe your (whole) procedure (and formulae) in a very detailed manner?