When you stop using sulfate shampoos, it often takes a while for your scalp to adjust to the change. It will continue to produce oils as if you are still using drying ingredients for a while, and your hair might feel greasy til your scalp realizes that it can slow down oil production.
When I first started I was using a very heavy conditioner to co-wash, and I also didn't spend enough time massaging my scalp and rinsing. I found that fixing those things and applying leave-in to my length and ends and more or less avoiding my roots helped a lot.
Also, some people just find that using a mild (sulfate free) shampoo once in a while works better for them.