I know two people who are having this problem. One has a Mosquito and the other has a Flying Squirrel (which is a pretty cool little yo-yo).
The problem they are having is that their stings are slipping off their bearings and getting stuck between the bearing and the spacer.
Their spacers are put in correctly. The yo-yos are snug but not over tightened. New string or old string it doesn’t seem to matter, it still slips off.
Has anyone had this problem and do you know how to fix it?
Actually, very many yoyos have slippage issues. Its a problem in the way the bearing lines up with the walls… a very slight mistake can cause this. There really is not much you can do about it, unfortunately.
that happens to my mini freehand hmmmm…
well i have had this problem too, but i found out that you jsut not tightening it tight enought, but you dont want to tighten it to much or it will break the yoyo, so kind of need to test were your yoyo wont fall off, and etc., thats all the advice i can give, good luck ;D
my friend bought a kk so this wouldnt happen,but if you fell like you dont need one you cant do much about it,sorry 
I was looking at his Flying Squirrel again today and we noticed something. We compared it to my Throw Monkey, both Duncan’s, and there is a difference.
If you take the bearings and spacers off the axle and pinch them between your fingers (spacer-bearing-spacer) there is a gap.
On the Throw Monkey, which has had zero problems, the gap is very slight and the string can’t come close to fitting. However, on the Flying Squirrel the gap between the bearing and spacer is just big enough that the sting can go in the gap.
It looks to me like the spacers weren’t machined out right.