I, too, have had some problems with Xtol (and T-Max films). I started using it a few years ago, with replenishment, and it was very consistent. After a while, I noticed that my film contrast seemed to drift up or down a bit, but nothing that a bit of adjustment in times could not handle. It is only in looking back that I realize that the changes usually could be correlated with a new 5 liter batch and that replenishment was masking or buffering and sudden changes.(posted 8632 days ago)I made a switch to a Jobo processor and decided to use 1:1, with 100ml stock solution per roll. Things went great for almost a year, including times matched with the developing chart. One day I mixed a fresh batch and contrast dropped. I figured it was my problem, mixed fresh and had the exact same result; I even tried inversion agitation (still 1:1, with 250 ml stock per 120 roll) but no improvement. I worked with this, talked with some people at Kodak, exchanged film and developer with them, etc. They said the developer was within specifications and nothing was wrong.
It seems that my problem has been variations in batches of Xtol as they come from Kodak. I now try to make sure that the 5 liter pouches all come from the same box/batch and test. It also seems that there is definitely some problem with Xtol that Kodak needs to both acknowledge and address. It is an excellent developer.