Probably because you've been messing with the flower nut and the clutch adjustment. Unless you fully understand what you're doing you should NOT loosen or tighten the flower nut. Doing so is some of the worst advice given on this forum to new builders.
Do a little research, use the keywords, 'clutch adjust' and you'll have plenty to read on the proper procedure for getting your clutch to work right. http://www.google.com/cse?cx=partner...all-35244.html