It seems like the extension and retraction duration of pistons is inconsistent. Redstone contraptions that theoretically should work, sometimes work and sometimes doesn't. It might be a problem with redstone ticks in general but i think the problem is with the pistons.

P.S. pistons in PE and win10 take around 2 ticks to retract/extend instead of precisely 1 tick, like in the Java version of the game. That makes most Redstone contraptions much bigger then they should be and its pretty annoying.