Two other possibilities, beside those mentioned come to mind. Rives BFK is
generally NOT a good paper for cyanotype. BFK is buffered (sodium
carbonate added) and cyanotype likes a slightly acid condition (for which
reason do NOT store your precious cyanotypes -- new or vintage -- in
buffered material).
Second, you haven't said how you dried the coating, but if you've been
extremely hardworking & dried it industriously with forced heat before
the emulsion has had time to sink into the paper, it can form a skin on the
surface which simply washes off in the water -- especially if the BFK
had a gelatine size, which you didn't mention. (A student of mine had this
mysterioso effect with VanDyke Brown; it was a couple of years before I
managed to retrofit the explanation. She didn't forgive me.)
It also occurs to me to ask about the condition of your potassium
ferricyanide. The book says it should be "ruby red crystals." We rarely
see a jar of true ruby red any more (certainly not from Kodak), but at
least it should be a pretty red orange. If the crystals look
yellowy-orange, and absolutely if they've got yellow lumps (like a sulfur
mine, or whatever), prints are pale -- though you will get an image.
One of the merits of cyanotype is the (relatively) long scale and delicate
detail, much of which are lost anyway with Rives BFK and its coarse
texture (not to mention the little white flecks BFK is prone to).
Besides the aforementioned Stonehenge (relatively smooth yet absorbent) try
a one-ply Bristol, a kid or plate-finish drawing paper, Fabriano
artistico, or any rag-content writing or drawing paper (like a
rag-content typewriter paper).
Hope you'll fill us in when you solve the problem. A recent
supplicant to this list -- who shall be nameless -- got a virtual
encylopedia of suggestions & then never supplied the denouement. We're
still in suspense.......too cruel.
Judy