Don't let this discourage you. There are a lot of factors at work here... First of all, you may have done nothing wrong at all. I popped 8 autos last spring and all of them grew into nice big plants other than my northern cheese haze. It stayed small and didn't yield anything really. It also produced a few immature seeds. It was germinated at the same time, in the same way, in the same tent as all the others. Give it another go... Autos are something that has been "bred by people" nowhere in nature will you find big potent autoflowers that produce good yields of prime bud. Every now and then you may run into an auto seeds that doesn't "auto flower" or one that is just an ugly little runt that was a waste of time. Using reliable proven genetics is a good way to avoid this but it's still not a sure thing.
I say finish out your current grow and see what you end up with... Then try again. If you get the same results maybe try a different breeder or a different strain or even a different seedbank and I'll bet you see an improvement in results. Autos aren't hard... You can get good results with minimal effort. In my experience you can get ok results with plain old organic miracle gro organic potting soil although it wouldn't be my first recommendation.