Are you drinking lots of liquids? The way magnesium citrate works is that it draws fluid into the intestines from your body tissues to dilute the salt content of the mag citrate in your intestines, which then makes your stools liquid. It also takes awhile- I don't have constipation problems but it takes about 6 hours for it to kick in when I use it as part of a fasting/cleansing day.
I've had the same problem. I tried taking that too and it didn't help til the next day. But it only helped a little and then I was right back to the constipation. Also, it gave me a bad headache, bloated and sick feeling. Not to mention that it tasted really nasty. :-(