Perhaps your chosen supplement isn't supplying enough? Also, maybe try getting fiber through other sources, like whole grain bread or pasta. I know 2oz of whole grain Barilla pasta has like 8 grams of fiber in it. I have fiber granola bars (Kellogg's Fiber Plus) that are only 120 calories, and have 9 grams of fiber. Also they have Thomas English Muffins that are only 100 calories but 6 grams of fiber. I believe this "kind" of fiber isn't as good as the kind you get from fruits & veggies, but it helps.
Also, look at the kinds of fruits and veggies you eat, and maybe try switching them out for ones that are more fiber rich. Here's a pretty good list: thyroid.about.com/od/loseweightsuccessfull