Rootbound?
I've seen many cases where the root ball gets so tight from crowding that water flows around the roots without much penetrating to the inside. Whether weed or anything else, I just get a thin rod, dowel, screwdriver, etc., and use it to break into the root ball and loosen things up so water can get to the majority of the roots. Sounds drastic, but I've been doing this for literally decades and it does not harm the plants. A little "root trauma" is better than no water, right?