That's cause you're not getting the right piece. Order a few of the ones that I posted in the tutorial online. My pieces don't screw all the way down but when you add both of the washers then it screws to the washers. Certain fittings are not meant to be screwed all the way down and they are tapered. The Electrical Grey Pieces are meant to screw all the way down but the white ones aren't. Each company has a different type of taper but the ones that I listed in the RDWC Build Tutorial is the one that I find that works best. All you have to do is go on google and do a search for the product that I listed. Then it comes right to your house. I'm about to order thousands of them so if you can't find them, you can get them at the store that I sell the washers at for a huge markup.
I see what you're trying to do. I'm not qualified to speak on vertical type grows. Also the way that your system in your design flows seems like it could be better. I know you're trying to hide the controller in the corner so you have access to it and other things because your space is so small but I like 2 connections to my controllers. I'm not sure what size plants you are trying to grow but I have a feeling you're trying to grow bigger plants because you need to get 5 pounds with your 9 plants. So you would need bigger buckets than the ones that I use and bigger pipes. Your 2" pipes would most likely end up getting clogged by roots and it would be very difficult to access all your buckets. The other problem is that because you would need bigger buckets at least twice the size of mine, you would need air pumps twice the size which would be running 120 watts 24 hours per day. If you had buckets that held 6 gallons of solution, you would need an air pump rated at 120 Liters per minute like an Alita AL-120 which would cost you about $400. If you used bigger buckets that that then you would need an even more expensive air pump. These aren't things that I would skimp on. My design takes all these cost into consideration as well as no more than 14 day VEG times to maximize a crop so I can get away with the 1.5" pipes and 5 gallon buckets that only hold 3 gallons in each bucket. It's set up for horizontal growing though. That's why it's hard to tell you what to do because you probably have your own ideas of how you're going to run this with no air pump or something like that. Not really sure. I'd personally rather relocate to a space that works instead of spend all that money to make something work alright.
