Might be overfed? Or, was this picture taken during normal dark hours? It's not a happy plant by the looks of it, if not. This could be the cause of slower growth.
take that as a grain of salt... photos are not perfect. if it is glossy, leathery or super smooth on bottom of leaves, back off on nutes for a bit. if you've been feeding from beginning, back off a bit. the soil is likely building up nutes.