Hi Jack, wild guess here, but it sounds so much like the problem I was having. The solution for me was to keep hold of the end of the string in your non-throw hand until the last moment, if you let go too soon, the string is free to slip around the tip without regenerating any spin. Also, you can pull up on the string much less aggressively because you get better control.
Anyways, good luck. I'm still not very good at it, but it's fun to practice.