hmm well it sounds like too much water to me. perhaps they just majorly wilt because of that, which makes you give up on them, thinking they're dead. i would suggest doing everything the same but get rid of the heating mat and get rid of the water in the bottom of the tray (still water ur cubes tho). if you don't think it is humid enuff in there for them, put a bowl of water in the dome with them. you should also be opening the dome to let them air out at least twice a day, for about 5 mins or so each time.