Fastbuds Pineapple Express Auto, RDWC run

It could be a huge range of reason why you didn’t get much root growth. Proper distance to water, temp of water, oxygen levels in water, pH of water all come to mind. If you’re saying your pH, temp and EC were in range then my guess is that it would be one of the others I’ve mentioned. I always ran massive air stones in my buckets and my roots got massive. I also used a product called VooDoo as well. Hope this helps.

did you leave a space between the water and the bottom of your pot or basket ?

I'm personally not a fan of the recirculating drip setups. I don't like dumping all that moisture at the base of the plant. You get plenty of moisture just allowing the airstone to mist your net pot and a drier base help promote searching roots. Autos also carry a higher probability of genetic variation, good and bad, so a stunted runt isn't uncommon at all.