You mean that what normally can last six months tightly capped in a bottle may some times just die after only 2 and a half weeks? The only way to find out is to mix up a new batch of the same developer from another source and try that on new negatives. Do you suggest I refrain from using the same stock solution on further negatives OR do I try once more just to see if indeed it is as you say SUDDEN DEATH? I must say that I cannot retrace my steps and find any reason other than what you propose to explain what happened. What about a stock solution not mixed into distilled water properly to make a working solution? How much stirring is needed to ensure adequate stock goes into the working solution?