I had the same trouble, posted here, and Yoyoexpert Garrett’s tips helped me out a lot. I can get the slap triangle pretty consistently now. Here are some things I noticed when I started getting the trick right:
The slap isn’t much of a slap at all, at least not until you start getting consistent. I found that a much smaller throwhand slap motion helped me, especially if you don’t raise your hand to begin the slap. Don’t move your throwhand until the last minute, and when you do, it should be a short, quick downward motion.
It helps to know what the string’s doing during the slap, so if you don’t understand how the triangle is formed, try making a large, exaggerated slack motion. This motion may not help you land the trick, but understanding how the triangle happens was helpful to me.
If you’re not already, try pinching the string a little bit with your nonthrowhand to keep the slack open and easier to slap. Your hands should come together a little bit in midair before the slap, which helps the yoyo catch the slack.
Hope these help, and Good luck! It can be frustrating but once you get it a few times, it becomes second nature.