2007 CRF150R. My son was riding, all was well. Then he said, when he shifts down, from third, the transmission goes in neutral. Will not shift to second or first. The only gears he has are 3rd through 5th.
Put the bike on the stand with the rear wheel off the ground and run through the gears verifying it's in gear by rotating the rear wheel. Make sure there's nothing in the shifter restricting its movement. Report back.