I had the same problem. After CRISPR/Cas9 I had to separate cells in single cell colonies for genotypization because my goal was to produce new cell line without chemical selection. I havent used your cell line but using 1/2 of culturing medium from my cell stock plate with 1/2 of fresh medium fixed the problem. Growth was slow but steady but in the end I didnt manage to produce wanted cell line because homozygotes couldnt survive for long enough