Hi

I'm working on cell culture, but I can't grow my HL-60 line. The closing date of the cells is 2019, in the study conducted on this date, the cells were growing at full performance, but currently it takes only 3 days to grow. I change the medium every 3 days, centrifuge at 300xg for 5 minutes and the cells do not settle to the bottom. What could be the reason why the cells do not sink to the bottom after centrifugation and what prevents their growth?

(Note: I remove the cells from liquid nitrogen and centrifuge at 300xg and 5 minutes for the first time in the washing process, and the cells settle to the bottom.)