I have been on questran for several years and it is an absolute godsend! I don't think I could cope day to day without it. It takes some time time to get used to the texture and taste, I have it in about 1 cup of water (I prefer cold) and just stir it as soon as I pour it in and keep stirring it constant. I only take one sachet per day, this usually keeps the diarrhea away for most of the day. I am fairly sure that it only took a few days to kick in, but if I miss a dose I am back to being on the toilet all day.
I am not sure about the halving of the dose. I have never done that, but I know when I have constant d's then I feel nauseated.
I would try what sybil suggested, go back to taking the full dose and if you see no improvement go and see your doctor. There could be something else going on.
Hope you feel better soon.