I had similar issue, so I put more pumps to each bucket. Downside to that was there was too much dripping and the splash would get all over the place and started to kill off the lower branching due to salty bulidup on the leaves not to mention it got all over my floor with salty splashing. Overall I was not a happy customer with my waterfarm modules as they don't give you complete control over nute balancing unless you don't mind the hassle of taking the buckets apart which is fucking hard once the plants start getting to a larger size.
I had heard one guy say that you can use a wetvac inside your bottom res to suck all the water out, but even so you're still going to have to put in a lot more work than they are worth. I've got an 8 pack now sitting in my garage never to be used again.
Not trying to be negative, but these were the worst systems i've ever used.