I've used a variety of cuts, beef cheapy steak has been the recent offender.

Do you put your liquids in cold...does it matter if it goes in hot (like if you brown the meat and then deglaze the pan and put that hot liquid in)?

Interested that you both start it on high then turn it down...what happens if you leave it on longer (eg put it on at 8 in the morning when you leave for work & then don't get home til 5.30)?
That's weird about your meat coming out tough. I always use the cheap cuts and they come out so soft.

If you are gone that long make sure its on low the whole time or you'll end up with mush. I have used hot and cold water, it doesn't seem to make a difference. I would maybe make the cuts a bit bigger for that amount of time.