Do fittonias like to be root bound? I know some plants do, and mine seemed a little happier when it was.
Certified GKH Gardening Expert
They don't, necessarily, like to be rootbound. They don't like to move up to too large of a container too fast, though.
Potting them up too large too fast will often result in overwatering, which these will not handle. The new container should only be one or two inches larger than the original to avoid overwatering issues.
Make sure to let any extra water out of the container after watering, and only water once the soil is completely dry down to about 2 inches. This will help keep the plant happy in the future.
These articles will offer some useful information: https://www.gardeningknowhow.com/houseplants/nerve-plant/growing-nerve-plants.htm