I don't run big boards, but my guess is that you aren't accounting for the height of the tow line with your depths. I assume with the 40 jets back 50' you are trying to achieve 20' of depth. If your tow line for your big boards is 5' above the water you need to account for that extra height on your let back. You may think you are fishing at 20', but probably are fishing closer to 15'. Someone with big board experience can chime in, but that's my best guess. I would add 10'-20' more on your let back length for your desired depths.