Pots too small? Delayed growth.

20 litre pots for autos is just about ideal, but at six weeks they should be already in flower if they are true autos. Delayed flowering is a genetic issue, not an issue related to pot size. Really isn't much you can do, other than continue your current care routine and patiently wait for them to flower. Problems like this is why I prefer normal photoperiod plants with their well sorted and well stabilized genetics. There seem to be too many variable and unstable autos on the market, perhaps the breeders are too keen to get "product" on the market and do not spend enough time fully developing and stabilizing their auto lines.