(Sticky) Pistons fail to retract under certain circumstances:

A piston pushing a redstone block up will not retract after redstone power is removed, even if a block is placed next to the piston to update it.

 It's worth noting that  all other directions have no trouble retracting.

In addition, the following redstone configuration results in the same problem, no matter the piston's direction. After placing the wool, however, the piston does update.

(Sorry if this is a duplicate of 108, but I couldn't find anything for pistons remaining after being unpowered.)