Sometimes it's caused from not applying enough up force on the shifter when shifting to 2nd. If this is the case it can usually be solved by adjusting the shift lever so it's neutral (normal resting) position is slightly lower than it is now. If that doesn't correct the problem then my first guess would be a bent shift fork. If yours has the adjuster built into the shift linkage, you can make the adjustment very easy and quickly. If it doesn't then you will have to pull the linkage off of the shift shaft and reposition it that way.