What happens if one reverser doesn't stow?
The system is designed to make sure the reversers completely stow.
At Cranbrook, the problem was the T/R hadn't finished the stow cycle when the air/ground indication went 'air' - which closed the T/R isolation valve. The aerodynamic forces on the T/R then pushed it back open (clamshell type reverser).
Since Cranbrook, the thrust reverser systems have been designed to keep the isolation valve open until the reverser has completely stowed and locked, and the actuators are sized to insure they can overcome any aerodynamic forces at high engine power.