If the developer was working fine when you first mixed it I believe you should discard the theory that there was too little phenidone. The amount you need anyway is miniscule and in my experience it would not go bad in two months in a partially full bottle.
Originally Posted by j_landecker
The two most likely reasons why the developer may have failed, in mind would be.
1. Somehow either one or both of the stock solutions was contaminated.
2. You failed to add the B solution when mixing the working solution, or the B solution is mislabeled and contains something other than potassium carbonate at the necessary strength.
The very thin negatives that you describe would in fact be what one would expect from using a very weak Stock B solution. This is the accelerator part of the developer that increases the working solution to the pH needed for development, and without it the catechol and phenidone will just sit there and do nothing.
Next time you mix a working solution check the pH if you the means to do so. It should be around pH 10.9 or so.