Andrew,
May be 15mins water was was too short so that you had problems, I always do 20 to 25 mins washing, shuffling the papers 5 mins intervals.
I can't find rising stonehenge here, so I am unable to test it.
May be there is a problem with the tap water, or with the sulfamic acid (additives perhaps).

Any case, I will paste my washing regimen from the initial post.
"I washed the papers (5 of them in the same tray) with tap water (chlorinated, not filtered, measured Ph. 7), for 20-25 minutes with 4-5 changes of water, and with each change of water shuffling the papers and bringing the bottom one to the top. When finished washing, I hanged the papers to dry in the bathroom."

regards
Serdar
Serdar, I'm making kallitypes.
I gave thorough washing, up to an hour, after my initial trials with the 10% solution. Results were much better. I compared the 1% and 10% prints side-by-side, and they are identical, at least to my eye. 1% worked with Rising Stonehenge. It may not with other papers.