I recommend keeping the Stock A solution for no more than six months, but in fact in my conditions I have used it from partially full bottles after more than a year with good success. And when I did have a Stock A solution go bad, after about 2.5 years, it announced itself to me by turning a very dark brown.

So in short I don't have an answer to your mystery question. But if the Stock A solution is clear I would look for other reasons for the thin negatives. Stock B, unless contaminated, should last for years and years.