pressure plate does not turn off

What I expected:
Pressure plate to turn 'off' when stepping off it.

What happened:
Pressure plate remains in 'down' position and does not toggle action.

Steps to reproduce:
1. Placed wooden [oak] door
2. Placed [oak] pressure plate beside it.
3. Standing on pressure plate opens door, but stepping off pressure plate does not close door automatically as it used to. Pressure plate 'clicks' but never 'unclicks'.

After trial, pressure plate remains in DOWN position and can no longer be activated unless removed and replaced.