I found out recently that if you get a reverser light on the ground, when you cycle the reverse thrust to try and clear it, you need to leave the reverser unlocked for around 30 seconds, then keep it closed for 30 seconds. Apparently you need to give the vales etc time in order for them to reach their full open/closed position.
Also, apparently the software/hardware that deals with this system and the unlock light has a 5 cycle memory, so doing it 6 times should work...
At least I was told all this by an engineer...