It would seem like it is an easy thing to do, but it is actually very hard. The thing is for me, my voice seems like it never sounds the same from one day to the next. The thing to remember is that if you don't warm up your vocals first, (I never do btw), my vocals sound better or stronger after a few takes. I would assume that the last takes you did the day before weren't the first of that day. For me, the last takes of the day are gonna sound more like the last takes of the next day. So what I am saying, for me anyway, I never expect the first take of the next day to sound like the last take of the day before, it just never does.... That make any sense to ya? So just make sure all your settings haven't changed, that you are the same distance from the mic as the day before, and then wait for the later takes to start sounding like the day before...
Curtis