Answer: Impatiens do take a very long time to come into bloom form seed in my experience. I usually allow at least twelve weeks! Based on your description is sounds like you have done everything right, although there is always a possibility that they were hit by a cold snap once you set them outside or grown with too little light or at too low a temperature early on. Any of these could cause stunting, as could being allowed to dry out excessively.
The only thing you didn't mention is fertilizer. Impatiens does best with a rich soil and plenty of moisture, and since the potting mix becomes depleted over time it is possible these are in need of nutrients. You might wish to try that if you haven't already. In any case, assuming they are healthy enough but just immature, they do grow best in warm weather and should pick up steam now that summer is here!
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