When I kept anacharis-I had the same experience-it would slow after awhile and then totally stop growing and reproducing-this is the main reason I don't keep it.
The Cabomba however, grow like a weed and I can't keep up with the growth-I have some that will grow well into the 2ft range-it rarely will have side shoots-I pinch the top and replant and the mother stem-keeps growing-even under the carpet of frogbit.
Are the trims from the plants growing well-if so and the mother stem isn't-remove the mother stem and replace it with its trimmed top.
Are you using any ferts, how much and how often are the water changes and stocking level-How many days or months has the tank been setup.
How old are the light bulbs
Stems can sometimes be picky and you have to use what works best in the tank.
The baby ferns-you can let them float if you have run out of places to attach them or start another tank....lol.....sometimes with successful planted tanks-you can end up with lots of tanks and containers full of excess plants due to plant growth.
Sorry, was low on time but now I can reply.
Whenever I trim a cabomba, I've never had the mother stem keep growing. There always has to be a side shoot (its attached to main stem tho). Eventually, they just run out and stop growing altogether, just like the anacharis. Trimmings do usually do well, but I don't get how I could remove the mother plant without getting dirt from the roots being pulled up everywhere. I also remember you saying you're not supposed to disturb the soil, but If i remove as many mother stems as I would need to, there'd be ALOT of disturbance. Remember, I'm doing the silent cycle with no filter.
Not using ferts, its NPT. Water changes usually about once a week (usually only about a gallon or two), mainly when I scrape the algae off the front glass, since theres already so much stuff on the ground. Ive got 4 male guppies, a mystery snail, and 2 cherry shrimp.
Light bulbs are close to 3 months old, which goes along with how old my tank is.
Idk, I kinda wanna scrap the NPT idea and just get a filter/redo stuff, but I can't do that until summer. I dont like how incredibly messy the floor looks, and I'm not* knowledgeable/skillful enough to know how to fix the problem of my slowed stem plants. Pulling the mother stems up doesn't seem plausible, since I tried that once and like I said dirt got everywhere cuz of the roots, my sand cap is pretty much the bare minimum too. I hate how everytime I trim a plant, it basically just sits there and does nothing, unless it has a side shoot but those eventually run out too.
My slow growing ferns, crypts, and moss are doing wonderful, probably cuz i dont have to trim them much haha. and I think those are what I'd stick with if I just redid the tank. By the way, the baby ferns are still attached to the main plant, should I keep em there or yank em off n float em. Will they come off by selves lol?