Kenda's leak like a sieve, get tires with better casings. Their mtb tires leak forever as well. I popped up a set of Clement LAS on friday night and raced them Saturday with only a couple psi lost over night. The PDX on my spare wheels will go a week without noticeable loss, the Mud 2 on my wifes bike are similar.
It seems odd that some people are experiencing problems with Kendas. In the last of the tubeless series in CX mag, the Kenda Kommandos were rated as some of the better tires to use. Which gave me the gumption to use it myself and once I got it to seal it's been holding great (1 1/2 months with very little issue). In fact in my second race on Sunday I burped it when I had a less-than-graceful remount, and then again on a small pothole-ish section on a downhill and I thought I was done. It held up for the remaining two + laps and I finished with 20.5 lbs in the tire.
That said, I have a Panaracer CrossBlaster up front that seemed to seal a bit easier. I also use the Stan's NoTubes setup.
One of the guys I race with is using the Kenda Straight 6 tubeless and it holds very well.
I was runing 29-30 lbs rear, 28 in the front. I clock a steady 180 lbs. With my experiences it seems I need a bit more pressure when trying to rail some tight corners. I'm going to try 33 in the rear and 30 up front.
Otherwise I can't complain. This tire combo hooks up good on off-camber grass and doesn't clog much with mud (both of which we had this past weekend).
Kendas have worked great for many of us, and it seems like you had good luck originally as well. I can think of three possibilities: you need more sealant, or you might be pumping them up more than you need to (if seated, there's no reason to go a lot higher than you'd ride - often it bursts the little seals), or the sealant has eaten away at the rubber over time and your tire is done.
Note that Kenda has released some tires with Sealant-compatible rubber, and thus that suggests they might have had some issues with rubber breaking down over time. We've tested some cx prototypes from Kenda with that, but can't really comment as to whether they've held up better over time.
Hope that helps. Good luck!