If you just pull the clutch handle while the motor is running on the stand it may not release, all of my motors would still spin the wheel. To check you have to apply the break, and you may have to give the motor a little gas to keep it from stalling, but it should release. If it stays stuck, the clutch plates may have dried out like mentioned above. I’ve had it happen once on a motor that sat for a year. The clutch discs got stuck together. I took the clutch apart, re-oiled, re-assembled, and it worked great.