When you grab that string so that you can see the triangle, you are putting your finger in from the wrong side. You want to go from the farther side, as opposed to the side closer to you.
I think you’re landing the yo-yo into the wrong string. When you roll the yo-yo off of the one-and-a-half mount, and after you curl your pointer finger in, you have to guide the yo-yo onto the string that is closest to you, not the one on the front.