measure how far 2nd gear engages, and inspect the shift drum for a worn channel at the top of 2nd. minimal wear can cause it to not fully engage. also, inspect the dog on the shift fork for wear. the fork can also be bent, but it is hard to tell just by looking at it, so if you cannot see any evident problem, it's most likely a bent fork. being off by as little as 1/16" can cause the issues, and you have to spin the gears a bit, sometimes, to go to the next gear. also, if the gear slipped out and started jumping over the dogs, that will bend the fork and round the cogs, so you may have to replace those gears too.