I thought you were using the coffee grounds 'and' oak bark ?
Oh well, coffee doesn't need a mordant, or tannin, but either or both help.
It's a pity that it's kind of too late for the last of the nettles or leaf fall, and too early for new strong growth, because with coffee that can give a muddy colour.
I think I'd buy a big pack of strong tea bags from the poundstore and brew that up as long as I could and use that in the dye liquor.
I know it's not free, but it's not break the bank expensive either.
That said, people are often surprised at just how much material is needed to give a good colour from some natural dyes. Kind of why the three classics, madder, woad and weld, were so highly regarded.
atb,
M