PA-23 pilot reported while demonstrating an engine shutdown and restart on a practical flight test; the engine would not restart. The pilot diverted and found after landing that the propeller had not come out of feather during the attempted restart.

Narrative

On Date I was conducting my AMEL Commercial Check ride out of ZZZ. During our time in the practice area conducting our maneuvers per the Airman Certification Standards (ACS). All mechanical functions of the aircraft had been functioning at a normal capacity and I had to demonstrate my engine shutdown and air restart under Task A of the Multiengine operations of the ACS. During this procedure all memory items and checklist were used to secure the failed engine. During the restart procedure we were unable to successfully restart the inoperative engine and [advised ATC]. It was decided that the check ride was now over and I made a positive transfer of controls over the other Pilot while I ran the checklist in our QRH for or gear extension. We proceeded direct to ZZZ1 as our intended place for an emergency landing. We were able to successfully land without any injuries or damage to the aircraft or any property. After proper shut down procedures we exited the aircraft and performed a visual inspection of the failed engine. Upon the inspection it was noticed the prop did not come out of feather. I believe the problem was due to that prop not being exercised in and out of feather paired with a weak starter. During our time in the air I believe both pilots followed appropriate actions and proper procedures to ensure the safety of flight and a well executed landing.