
that is a bigger question than you realize mate,... it could be so many things, and one would need a whole breakdown of your grow to figure it out,... pics in normal light are needed for symptom diagnosis though, which may or may not be the reason for smaller plants....
--warm temps, at lights off too are very helpful
--- monitoring and adjusting your inputs pH, feed and water (if applicable, depending on source and pH and ppm's of it),... Your in-pot pH is a MUST to watch, no feeds, organic or otherwise are going to do this "self-correcting" BS once in the pot... you will need a decent pH probe for that, something like the Accurate 8 or the tricker Blue Labs unit made for this specifically.....
Point is, if the pH goes off range, your roots hate it, certain nutes may lock out and the plant stunts,....