Bunched plants.

Lead helps hold them in place, but gravel alone should work too.
Are your fish digging the plants up?

Are you trying to stick them into the gravel by hand? That gets old really quick and they never seem to stay down. Try using some long tweezers to plant them. I have some large hemostats that work miracles with stem plants.
Just my opinion.
later,
ian

Wait a minute, you leave the weight bands on when planting? Can you do that? I've always just stuck them in the substrate...

I guess it's 'cause I don't want to buy tweezers, or maybe 'cause I like doing it by hand...

Depending on the plant, I take one to three stems, line the bottoms up even and hold them between my index and middle finger. I use these two fingers to slide the stems into the substrate, gently grab the stem with my other hand and slowly pull my fingers out, using the other hand to keep the stems in the substrate. While holding them in place, I pack the substrate back up around the plants and pack it gently. This has worked very well for me with Egeria, which, like Cabomba, does not root.

If the plants actually have a bit of root formation, I can do it one handed.

Well, all I do with Cabomba is wrap a weight around it, and drop it in the water so the weight doesn't show (like behind a rock or decorations or something). I dont push it in the substrate at all. This is easiest, since I have to take it out and prune it every two or three days, it grows so fast. I do this with most of my fast growing stem plants, and never had any problems.