Did you start the plants in the 5 gal buckets? If so, the roots typically just shoot straight down and out drain holes, instead of expanding in the whole bucket full of dirt.
It is better to grow your plants in smaller containers and step them up after they fill out the smaller container. So as example, start in 16oz cup until roots are all you see when you pull it out of the cup to transfer into a half gal or 1 gal container and repeat this process with 3 gal and then finally into 5 gal for flowering. If you google this method you'll find more in depth reasoning as to why this makes sense.
Plus if you step up container sizes slowly, you won't have to fertilize nearly as much because each time you transfer the new dirt replenishes nutrients. Cheers.