David, IME, with Vandyke and Argyrotype kind of sensitizers, it's best to use them up within 3-4 months; to be on the safe side, just mix and/or purchase the amount you're going to use up within 3-4 months. Also, the coffee filter that you used may have caused contamination; use specialized / laboratory grade filter papers for filtering chemistry to be safe. (OTOH, as a note: I haven't had any problems with "Melitta" brand coffee filters up to date...)

Regards,
Loris.