gear shift problem

[nfn]

Hi
There is a Gz-250 motor bike

The gearshift is not working properly
Cold may be perfectly all right to convert.
But if the engine is warming up 1-2, 2-1, 1-0 shift is hardly possible, try several times you can flip through, do not be pushed into gear. as if it would stretch.

What could be the reason.n.