It has a certain finesse to it. Just like GregP said, if you’re going too quick, it most likely won’t work. It’s a slower kind of motion to get a nice big loop. I personally like to use two fingers to pinch with my thumb because it provides a lot more control and stability. If you’re doing it right, getting a solid wrist flick, you should be able to see the loop form and watch the yoyo fall right into it. I also like to put my throwhand slightly farther from my body than the other, so the string loop can more easily form around the segment between the pinch and my throwhand.
Also always have good string tension. Number one rule. Good string tension always helps.