If you have shortened the string by tying it on the dice there is now not enough string for it to throw right. If the string is too short, it will over-rotate and come out sideways or upside down even. To know for sure though, we need to know if you are throwing the top correctly in the first place… is the tip up when you throw?
The reason to use the button is because if it is attached to your finger while you wind, the string becomes very twisted, so it’s just easier to use a button. Make one out of a water bottle lid or something, and try to restore the string to its original length. You don’t want heavy buttons like dice, because the weight pulls it away from your fingers while playing. No es bueno when you’re just getting started.