when you power 2 pistons which are above the other one (like on the screenshot) so they extend, and then unpower them, they retract both so they are as at the beginning so like this:

P = Piston, S = Redstone-Signal,A = Piston Arm,_=Empty:
__ ---> A_ ---> __
_S ---> PS ---> __
PS ---> AS ---> P_
PS ---> PS ---> P_
how it should be:
__ ---> A_ ---> __
S ---> PS ---> P
PS ---> AS ---> __
PS ---> PS ---> P_