pistons delete themselves!

i have a contraption that pushes two pistons with a piston through a piston 1 by 1 door in the floor and then the upper one pushes the third one forward destroying the lever that was pulled then it does this in reverse, however in the new snapshot it deletes the upper two pistons completely this happens when the pistons are retracting they disappear into thin air, my contraption is symiler to that made by the youtuber sethbling (useless lever contraption) not the same though as i needed it smaller