Today I produced two of my first VDBs. I am using no contrast control just trying to get the hang of it. My first image turned out very nice, and even darkened a bit when placed in the fixer. The second image place in the same fixer bleached out in a matter of seconds. I am using a 10% Sodium Thio. Why would the first image darken, and the next one bleach? The only thing I did different was the amount of exposure. The first image was exposed for about 4 min, and the second for only about two since the calotype neg was a bit thinner.
That sounds very strange. Firstly your Sodium Thiosuphite solution is too strong, try 2%. But perhaps if you can say more about your entire process steps we can help.