Without being able to see and mess with the bike a bit, I would guess that you have a shifter pawl spring broken in your transmission. It's either the return spring or the spring on the pawl itself. To test the spring, start from the "neutral postition" of the shifter. Regardless of the gear the bike might be in, when you are not pushing down on the shifter or lifting up on it, it is in the neutral position, where it normall sits. Now, using your fingers push down on the shift lever until you feel the transmission start to shift, you feel a resistance. Stop right there, do not shift the gear but allow the shifter to return to the neutral position. Now, lift the shifter with your fingers until you feel the resistance of the transmission getting ready to shift. What you are feeling for is the resistance of the shifter return spring. Does it feel the same in both directions? If not, the spring is broken. If it does, then the problem is something else.
Still if the transmissio will not shift into a particular gear, something is wrong inside the transmission and you'll have to go into it to find the problem most likely.
on Jul 16, 2017