I agree with the above advice on most counts. You should definitely have her stool checked for other parasites to be sure. It could be something other than worms, like giardia, which wouldn't be solved by the typical dewormers.
It could also be a grain intolerance. My second kitten had chronic soft smelly stools when I first brought her home. It continued for about 2 months, all parasite tests coming back negative. The only thing that fixed it finally was switching her to a completely grain-free diet.
The only thing I sort of disagree on is that using only one brand of food is best. For some cats this may be the case, if they are particularly sensitive to a number of different ingredients, but I personally think sticking to only one kind of food comes with potential problems: fussiness about food, boredom with getting the same food every day, lack of adaptability to different foods if you do ever switch, getting too much of or not enough of certain ingredients, and the potential for developing allergies or intolerances to specific proteins or ingredients if fed the same ones all the time.
When introducing a new food for the first time, this should be done gradually to give the cat time to get used to the new food and avoid digestive upset, but beyond that, I think rotating brands/flavors from day-to-day with wet food, or mixing multiple kinds of dry together should be just fine as long as the cat doesn't have any sensitivities to any of the ingredients in the foods you're offering. My cats are currently on a mix of 2 dry foods (although I'm switching them to just one for convenience) and get different flavors of canned (of 3 different brands) on a daily basis. The kitten with the sensitive stomach (Athena) does just fine with this as long as there's no grain in her diet.
So you definitely may want to try eliminating grain from their diet. It could be the grains in the ProPlan, Nutram, or ScienceDiet (I assume that's what you meant by science plan?) foods. Lots of expensive foods will claim to be "premium" or healthy when they really aren't. It's the ingredient label that tells the most accurate story, rather than the price tag or product claims.
I've heard good things about Orijen, so that's a good one to keep in their diet. Other quality grain-free foods are Wellness (certain flavors aren't grain-free, so check the labels), Merrick (all the Before Grain flavors are grain free, and some of the Gourmet Entrees ones are), and Blue Buffalo Wilderness (the non-wilderness varieties do contain grain), and Evo. All of these brands make grain-free dry and wet options.