Sorry, but I always chuckle when I see a pond plant that is supposed to have its leaves stay dry. I mean, really? How does that even work? What about when it rains?
Anyway, your pond is still very new, so that's going to make it a bit more of a struggle for plants to get going well. You simply aren't going to have the nutrient load for them to feed off of as you will after the pond's a few years old. Is it in an area where the roots can anchor to a bit of gravel? That might help, but if it is totally free floating, then it's got to depend 100% on the water & that could cause it not to flourish at first. (please note, I'm talking in generalizations here, I've no personal experience with frogbit, so I might be way off base)