Yeah, its looking more like a small issue with the bottles to me. If there was a small amount of oxidation cause by the bottle, and after 1/2 the rolls in 3 months say the level of oxidation increased 2% each week well, yeah, you'll get to a full stop under pretty quick. I use a glass gallon jug with rubber stopper, and after about 30-40 rolls I switch from my starting numbers to an intermediate, lets say adding 10%, then after say another 30 rolls switch to my main numbers (say another 10% and that is that for as long as I care to keep it going. 150 rolls over 6 months is not bad, you are certainly a good and suitable canidate for the replenishment scheme, so I'd work at figuring this out. If this is your first try at doing Xtol replenished don't give up, you've certainly learned a lot so far. I remember starting down this path, and I pretty much thought I'd change the whole batch at 6 month just to be safe, and after a while I got to a year or more pretty easily.