redstone block keeping piston arm extended when unpowerd

I set-up a flip-flop circuit with a sticky piston and a redstone block on the piston.
the piston was facing so that the arm would extend upwards, thus pushing the redstone block upwards.
i activated the circuit, the piston extended as normal but when the circuit was de-powered the piston arm remained extended. circuit work as normal with other blocks (tested with sandstone)