How are you storing the solutions? In full glass bottles stored in a cool place? Self oxidation will affect developer activity with time. Storing part used first and colour developer for more than a few days is usally a no-no. Partially oxidized solution causes the not affected part by the film action to be consumed by being exposed to the partially oxidized stuff. l don't keep it around for longer than a few weeks, and the second week is used for non critical stuff.
I usually store up films, and then do the developing on as fresh a mixed solution as I can.
Kodak's z-119 guide on E-6, chapter 9 is to increase first developer time by 30 seconds after the first two rolls of 135-36 have been run per 500mL when not replenishing to account for the fact that you are not replenishing, and the activity of the solutions are now partially depleted. Koak actually says toss the stuff after 3.3 rolls per 500mL if you pro-rate the 5L volume data thay give.
I have used 1L to process 6-8 135-36 rolls successfully when I process one batch of 4, and then the second 2-4 the next day.
And yes, e-6 first developer sets the density of the slides, and it is very finicky. My first developer concentrate expired in 2006, so it has lost a bit of its original vigour (but I do have an almost full 3.8l of concetrate), so my standard first dev time with this stuff is 7' at 38,
I ran a second batch of two rolls of 135 -36 from the Kodak 6 bath e-6 mixed and used to do 4 135-36 rolls at once on monday night on tuesday night.
I went with the process when the developer was at 37.6 (I was impatient, and went for 7:20.) The slides are a bit too dark. I have one roll shot in the same light as one of the 4 rolls done monday night to compare to to prove this. To temp control I use stainless steel reels and tank, and keep them in a water bath. My water bath is a small lunch cooler that holds an aquarium heater and circulating pump. Chem bottles go in first and then I fill it with sufficient water to ensure the heaters thermostat is below the water line. The lid goues on loosely ( power cords prevent a really good seal ). The heater has had its bi-metallic thermostat setting dial stop bumps chiselled off to let me wind it up to 38C, and then the top sealled with electrical conduit sealing mastic putty to keep condensate out of the heater top.
To troubleshoot you may want to meter and shoot a grey test card as the first shot after you load the camera with each roll. Then you can isolate out the exposure from the developing issues.
Metering with reversal film is quite a bit more demanding than standard negative film. Plus you need to reverse the thinking of what underexposure does versus when you shoot negative film.
I hope some of these comments help. Z-119 is on th web, and a thick read, but all you need to know is in there.