True the First Officer could have communicated through the door to the Captain about a problem with the door lock IF that was the case BUT also, that could serve as a ruse. What “pilot type things” indicate a deliberate act?
Go read the information from the Prosecutor. He gets into specifics.
If the door had electonically malfunctioned he could have gotten out of his seat to open it manually. The door lock mechanism can easily be depowered, with a guarded switch, from inside the cockpit.