seems to be initially overwatering, you possibly have experienced underwatering at some point to :/... you don't need water running-off when watering in soil, coco is different as it doesn't hold water like soil does and flows right through coco, try determine how aerated your medium is as soil doesn't like too much water, for smaller pots water fast at the base of the stem/middle of the pot allowing the water to splash/pool covering all of the soil so to equally drain through to the bottom of the pot,careful not water excessively long, you shouldn't see runoff in soil.
water slowly around the edges and the middle in bigger pots/ground, being really careful to not overwater, use appropriate sized watering can for soil so to not drench (again don't matter for coco) especially when just transplanted into the larger medium.
Next step is crucial, you have a window lets say around 24hrs where your medium will be dry and will finally get the oxygen it has been dying for, if you do not give the roots the oxygen they need they will be fucked (watering early/Overwatering) and takes time to re-grow the roots that were damaged, if you hit this dry period/sweet spot your plants will love it and you will very quickly understand and appreciate the results of giving the medium a full dry out when hand watering like this, a few things to note.. in coco it is hard to overwater in a single watering as it doesn't hold much water, soil you can fuck up just by giving it too much water in one go, however nearly all new growers will still overwater even in coco because of one reason which is crucial, let me state this again Let the medium fully DRY out..... the reason most people fall victim to overwatering is because they water TOO SOON and don't let the medium dry out.
So let the medium fully dry, not just the top soil/coco, then water when you feel its DRY (looking, feeling the medium and picking up your pot can help you determine this)
that's why I said around 24hrs after being fully dry is when you might begin to worry of the plant wanting water but around this time and before this is perfect and what
the plant wants and craves for, it wants oxygen..
if your medium is bone dry for lets say 30+ hrs then you will start to see leaves really sagging and wanting some water, so watering and drying cycles are extremely important, you are growing roots essentially the growth above the soil helps translate your roots health and problems. watering properly shows you fully healthy plants with leaves 'praying' towards the light, if you miss the watering sweet spot, forsure you will not see your plants do this until close to or maybe far past your next watering, of course depending on how badly when you decided to water..
One more time... let. your. medium. dry.
If your temps aren't completely out of range that's a start, humidity is important but shouldn't cause abnormally slow growth like you have seen but low humidity a long with too close of a light or slightly hot temps and a fan blowing air across canopy is enough reason to understand slow growth (still your plants seem small must off fucked the roots a lot I'm guessing mostly overwatering..)
especially when plants are small/weak, don't have a fan blowing across them unless you have good/high humidity, even with high humidity you don't want your plant or leaves blowing around much/ directly in a fans path as it can cause wind burn
Let them do their thing, cannabis is one of the fastest growing plants, don't try do to much don't stress, relax, give the plants some time, be patient keep it to the basics and just understand how important watering actually is.
You have probably overwatered and could have possibly in compensation waited too long to water another time and experienced under-watering them as well, so don't think too hard what you may or not have done cause you probably done many things far from perfect it would be impossible to know everything you did to stray from full health and vigor.
find that sweet spot without letting it getting bone dry for longer than a day but don't water too early both is so important , remember as your roots grow this sweet spot (time to water) will come by much faster as they drink more/faster.
Whatever the case now you understand watering is extremely important, miss it by 1 day and your fucked water early by 1 day and either way your fucked if you miss this by a decent bit...
Become proficient at watering then you will see what's up....
its not a hard activity just very important to follow every time like clockwork
I can't stress this enough, water plants individually, well maybe this is obvious to you, but if you had say 15 plants you might try water some of them together... DO NOT water unless needed... do not choke your plants by watering early, they are trying to get oxygen and don't need water..esp if crowded Check regularly as to not let any dry overly long, it does happen as drying is essential as you just cant cut corners and water too early (even though it will stunt em, if you forget/don't water you have easily 2days more like 3-4 where you could get it back to life but it wont really grow/have vigor for awhile) at the expense of sounding silly I hope repeated some of that shit enough for you to take on board