As you put more prints through your solution did it turn cloudy? I have found that once the cloudiness sets in it still tones OK but you get this greasy film on the prints. I have found a long soak in a hot solution of sodium carbonate and then a long wash in running water usually gets rid if the film.

But I have found a way for this not to happen and that is to use a highly dilute solution of Viradon, somehing like 1:50 or 1:75. Toning takes onger but it works.