Vandykes, gold, unneven toning

I did a few 8x10 Vandykes on Arches Aquarelle, and the printing went extremely well, however the toning (in Tetenal Gold) didn't work as well; some parts of the print toned to pure gray as expected , and some other stayed dark brown...
And that after 1/2h in the toner, and I even tried heating the tray some to 'help' : no difference.

Do what went wrong ? I blamed my clearing at first, so I overwashed a print after the exposure and it had the same problem.

Could it be the paper itself ? The Vandyke mix (a couple months old) ?

Here is a scan. It looks oretty bad on this one due to the high key tones... the other, darker prints of other subjects are not really as marked as this one..

Maybe it's a coating issue? Not coating it evenly enough or something. That's the only thing I could think of in the circumstances.