What is your approximate location on our planet?

Latitude and longitude are very important. There some locations where one of every three bamboo, treated similarly, simply will not grow.

The seasol would probably help since they were already in pots before the replanting, so none to little root damage when transplanted. I noticed in the first picture it looks like you can see the size of the original pot by the shape of the soil around the transplant. If that is the case, then they may not be getting enough water since you have to realize, the size of the rootball, or original pot size, is the only area the roots are. It will take some time for the roots to grow into the new soil surrounding the roots, and the water in the original rootball can be absorbed quickly, and I've seen people under water their bamboos thinking that the soil is moist, but they are only checking the soil 'around' the transplant, and not the soil where the actual roots are.

Them being grasses bamboos don't really become root-bound. They can certainly fill a pot up with roots, become quite crowded but it's not the same as when an actual tree becomes permanently deformed (bound) by under-potting.

"As plants grown in containers mature, their developing roots eventually will run out of space. When this happens, the plant becomes “root-bound”. The roots will try to escape out any drain holes in the pots. In some cases, they will try to slip out of the soil and over the lip of the pot."

When the bamboo becomes rootbound, it will start to look sickly because it has run out of nutrients/soil and when you remove the plant from the pot, you will see mostly roots with little soil left. Generally when they get severely rootbound, and you water the plant, the water will run right through the roots and out the bottom of the pot, but you will be able to see roots on the surface before even removing the pot.

You are welcome. Keep us posted on their recovery.