I saw a video recently on the tube about painting with watercolors and inks and a lady mentioned to make your inks permanent:
1) wash the item/fabric to remove factory added sizing
2) do not use fabric softner
3) let dry for several days, she often waits up to a week before heat setting. Some how it helps the paint/inks "set up" first.
sorry I don't have a link, it was one of those late nite can't sleep so I'll surf the web moments.