I have a zz plant that I had been unknowingly overwatering about 1.5 years ago. The top 2 inches of soil would dry out prior to that. The plant was mature with multiple large stems. I noted that the leaves were yellowing. Removed them gently from their pits and found a horribly rotted root system. I followed some suggestions and removed the affected roots, thoroughly rinsed the rhizome, didn't treated with anything and repotted in clean soil. I also divided the rhizome which was quite lage into three pieces. This was based on the condituon of the rhizome at the time. Since then, I have placed them in a smaller pot, as I felt that was part of the issue, and converted to a quick draining soil. Since that time, I have carefully watered the plant and avoided letting the soil retain too much water. There has been some small shoots since then, two leaves max. I went to repot the plant as it had been a year in their existing pots. There is no new root growth. Is this normal? Should I be concerned?