Sorry to hear you are having trouble losing weight...
Fiber supplements usually don't have any interaction with alli. However, because the alli meal plans already have about 25 grams of fiber (the recommended amount for adults), a fiber supplements may cause digestive problems such as constipation, gas, bloating, etc. I will recommend you stay with the meal plan, which already have sufficient fiber. Here is more information about fiber supplements you may find interesting:
Form: The American Dietetic Association by Roberta Larson Duyff, RD
"When it comes to fiber, you can overdo a good thing! Too much fiber can move food through the digestive tract faster than some nutrients can be absorbed. By eating more than 50 to 60 grams of fiber a day, your body may lower the absorption of vitamins and minerals, among them zinc, iron, magnesium, and calcium. An excessive amount of fiber may cause gas, diarrhea, and bloating. By filling up with too many high-fiber foods or fiber supplements, you may not have an appetite for other nutrient- rich foods.
How might you get too much fiber? By eating a lot of bran, very-high fiber cereals, or perhaps using a fiber supplement in an eating plan that already has plenty of vegetables, fruits, and whole grains." The key eat your recommended amounts of fiber (25 grams per day - already included in the alli meal plans) and drink plenty of water - new requirements call for a minimum of 13 cups per day!
Also, make sure you are login your food intake with the nutrition logs. This will help you make sure you are staying within your recommended calories and fat grams and not exceeding them.
Hope this helps; but let me know if you continue to experience any further difficulties losing weight,
Sylvia Melendez-Klinger, MS, RD, LDN, CPT - registered dietitian