I am trying to do a lentiviral transduction of U87 cells. I propagated viruses with PEI in HEK2923T cells. Although the protocol I used was previously optimized by our team, I failed to infect U87 cells. The only difference from our other experiments is that we're doing it with younger U87 cells. The dose I used for the blasticidine selection is 3 µg / ml. What are your suggestions for a more effective transduction?

Thanks