what size pots are they in? rootbound plants will eventually lose all the bottom foliage. also a 6ft plant is WAY to big for indoors. you are wasting alot of time/space growing a plant that big indoors unless you have some serious lighting, which i don't think is the case seeing how big the internodal spacing is. you will only have decent bud on the top 1/3 of the plant at most, the rest will be pretty much a waste of space. it might just be that the plants are way too big for the light and are shedding the bottom foiliage because of lack of light. the plants might think that there is no need for alot of fan leaves to absorb light for photosynthesis if the light isn't available to absorb, just a thought. i have no experience with problems associated with plants growing too large indoors as i have never let indoor plants get even close to 6ft tall. i grew scrog style and only cut very few fan leaves and only ones above the screen that were shading budsites, i never cut anything below the screen. fan leaves are like gas tanks as well as foodmakers. they store energy as well as make it, thats why you shouldn't cut off any more leaves than is absolutely necessary. hope this helps.