How big is your grow space ?
I believe your light is recommended for 3 x 3 ?
Have you ever measured par ?
Might just not be reaching their full potential... Lots of variables, ie., medium/pot size/nutrients/ventilation, etc.
As to when to harvest, I like to see the majority of the pistils turn color And recede.
Then I will look at the trichomes on a number of buds, I aim for the majority to be cloudy/white. A few amber doesn't hurt !
Nothing wrong with the potency of auto flowers.
I "thought" I would see if what was left of the King Gelato auto in my bong was done, it was two hours later before I could start to cook supper
Cheers