Wont Go Into 4rd Need Help

[XCHSER]

Hi i have a 125 lifan engine and when u click into third gear some times 2/10 it lags like a second then jumps into 4th? any ideas what it is clutch gear box? Anyone had this problem like before or now how to fix it? Its like it hits netrul then snaps into gear.?
Any help would be much appreciated.
xchser

 

[ducar.r.spec]

ull find that u might have a snapped tooth on ur 2nd and/or 3rd gear causing it to jam and then jump