I can shift in to any gear while my dirt bike is not running... I have even started it in every gear... and if I start it in 3rd I can ride out 3rd, 4th , and 5th... but if I down shift to 2nd I can't get it back up in to 3rd... if I start it under 3rd gear I can only shift in to 1st and 2nd... I don't know anything about the bike it's a 06 shucks jianshe/yamaha.buffalo DT150 china bike... I bought if for $50... I'm just trying to figure out why it won't make the gear transition whiles its run... Any ideas please shoot them my way .

Either the syncros are messed up or you could have a bent shifting fork. Anyway you look at it the case needs to be split open and the transmission needs to be examined. If your not sure how to do this take it to a professional.

If this has a manual clutch, try tightening up on the cable, but not so much the clutch slips, enough to get good dissengagement.
Adjustment is at clutch lever.

Thank you both... it's a bent shift fork with a weak spring and a over adjusted clutch cable... OH yea just so everyone know the china shucks bikes use Honda xr 200 motor parts... only difference is the bore size...