This hasn’t happened a lot, but enough to make me concerned: when I throw a sleeper every now and then the yoyo looks like it just detaches itself from the string. The yoyo then falls to the ground while the yoyo string just dangles there without any cuts. I do notice that the string isn’t as twisty as when I first got it, but instead two strings that are clearly separated. I use cotton string, so is it just the strength of the string or does the problem have to do with the separated string, and if so, how do i prevent untwisting it?
Use poly string
Your string is untwisting. It’s a natural result of throwing the yoyo.
For a start, take it off the yoyo and hold it by one end. It will tend to retwist in the right direction. Put it back on the yoyo and look at it closely to see what direction the twist is going. Give the yoyo a spin in the direction of the twist to add a few more turns. Try it and see how it plays.
Then take some time to learn how to adjust string with string tension tricks.
http://yoyoexpert.com/learn/017-intermediate-string-tension.html
And it doesn’t matter what kind of string you use - cotton, slick, poly. They are all twisted and will tend to untwist or twist more as you play.
Are you throwing left handed? If you are I’d suggest left-handed string.