After never having had a similar problem with any other developer, I quit using XTOL the first time this happened. I found that the date on the package was one month out of date. I don't want anything to do with a developer which dies while a powder in the original package.
I have used numerous developers, both packaged and self formulated, since and never had the problem. I will never try XTOL again. There are too many very good developers which do not have such a problem.