I always had problems getting clones to root too, I started using a 1020 tray and a dome and have literally gotten a 99.9% success rate since then. Most all of them have roots within 10 days. And the mothers health is a big factor as well like I've seen several people mention. I honestly think your trying to do too much, I wouldn't add anything to the water at all, not even ph up or down. Get you some jiffy pellets, a 1020 tray with a dome and mist the dome manually everyday, the first 3-4 days I will mist the plants but after that only the floor and the dome. When you take the clones, I don't think it matters how long they are, mine are usually about 3-4" long total, trim off all but the top 2 nodes of leaves and then clip those halfway down the blade of the leaves. It doesn't matter which strain,or if its indica, sativa or hybrid. Keep it simple and you will have success. The biggest factor for my turnaround with clones I truly believe was the dome and tray.
Good luck and never give up, that's a weak mindset.