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It is likely not simply an issue of using a “richer” media, but more so a better (i.e. possibly different) media that allows for more rapid growth of your cells so that the highest percentage of cells as possible are actively dividing at the time of transfection to promote DNA entry into the nucleus. There are many media developed specifically to support rapid cell growth and high density cultures and I would suggest screening some of the various 293 media available to find the one that works best for you rather than simply adding feeds or other supplements to your existing media. Gibco FreeStyle293 media and the newer Expi293 media are specifically designed to support high density HEK293 growth and transfection.