Have you checked for any leaks?
I am familiar with my own machine, but have not had experience with others. The article was more about how to go about shampooing the carpets.
I know the conundrum I face with my own machine is the annoyance that the dirty water tank does not seem to hold quite the full amount that the fresh water tank does, so it needs emptying before the fresh water needs to be refilled.
Possibly, your machine may have a dirty water tank that is a different shape or configuration than the fresh water tank, and it may only look to be a smaller amount. If the machine is picking it up, it's in there.