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.