Just checking here again... Well without hearing an answer to my question I would guess that you don't have enough substrate or gravel in your aquarium. I don't use Flourite and I don't know the dimensions of your aquarium, but I would guess that it is probably spread somewhat thin.
I have very large amazon swords in mine and I have once in a while had to pull them out for various reasons. I've never had a problem getting them to stay rooted. Tucking all the little rootings back in was an issue however. My gravel is about 2 inches in the front to about 3/3.5 inches in the back. (It is deeper in the areas with my very large swords)
If you don't want to buy more perhaps you can mound it up more around those plants that tend to pull out?