Need Help With Sagittarius

I'm noticing that where I've cut some sagittarius (due to algae growth), in certain spots it hasn't begun to grow back. Some parts have, but quite a few stay the same length and are again covered (including the pruned end) in the light layer of algae that caused me to prune it in the first place. Is there something I can do to encourage these stagnant sections to grow out? What has baffled me about this is the sagittarius are spreading both through runners and whatever that leaf-like structure that comes from the middle with a new plant, but my existing plants look ragged.