I havent seen this aspect of clearing kallitypes on any search so I hope this information will be helpful to some. I found that sensitized paper that was stored over night and exposed and processed the next morning would not clear under any conditions: new developer, old developer, more acidic developer, citric acid clearing bath. The unexposed areas under the mask would simply not clear. A newly mixed batch of sensitizer cleared completely in a couple of minutes in new developer as well as old developer. My guess is the sensitizer, over time, reacted with the alkalinity of the paper forming iron hydroxide. The timeframe in my case was something under 18 hours. Im using Lenox 100 paper and sodium citrate developer.