It doesn't seem that the doxycycline is very effective. Has your vet done a culture and sensitivity on the nasal discharge? As with most feline upper respiratory infections, there is usually a viral component as well, for which the antibiotic does nothing.I don't know what supplements you are giving, but a few homeopathic remedies you might try are Kali bichromicum, Hydrastis, and Pulsatilla. Echinacea, goldenseal, zinc, and vitamin C might be helpful. If would be best to have your cat examined by a holistic veterinarian.