Cell growth depends on factors like environmental conditions, Nutrients, and Handling. etc. Could you cross-check the nutrients you are providing and maintain the correct pH of the Medium with proper growing conditions?

If the cells do not grow, but are not dead, it is possible that they are infected with mycoplasma. You can test whether there is mycoplasma contamination. If so, adding anti-mycoplasma drugs to the culture medium can restore cell growth.

In our lab, we observed that NCI-H1975 cells sometimes exhibit low growth rates. To address this, I recommend using a medium containing 20% Fetal Bovine Serum and changing the medium every two days. This approach should enhance cell growth. Additionally, ensure to monitor the pH of your medium and test for mycoplasma contamination as previously advised.