By DS I mean dark slide (film holder).
I have used made up C41 developer that was older than 2 weeks, although this was a homemade brew kept chilled in tightly sealed bottles. I think the longest time between mixing and using was about three months ! I have never had an obvious problem with density (as judged by the numbers on the film rebate). I don't know about the commercial kits, but these devs are made to be robust.
I use C41 dev on a one shot basis. In my CPE2, it gets well shaken and looks horrible when it has done its job, so I have never been tempted to put it back in the bottle.
I know its obvious, but could your process temperature have been wrong ?