Reasons for non-functional piston?

[Joe G.]

What pressure is your regulator at? Solenoids can have unpredictable behavior below a minimum pressure.

[DampRobot]

Try taking the air lines out of the offending piston and triggering the solenoid without them in. If the air switches sides like you expect it to, you have a plumbing problem or an air pressure problem (or no problem at all). If the solenoid doesn't switch sides when you expect it to, you have a wiring problem, a code problem, or a underpressure problem (like Joe said).

[Mr. Mike]

If the wiring and program checks out OK and the solenoid does not switch the piston could be stuck in the manifold.