Yongnuo YN560 lll fires only on test

I have tried to fire (pc to pc) my new unit in a Nikon F and an FE. Also on the FE's hot shoe. And, also on my Graflex XL via the sync cable. At first I suspected the pc cable connections. I did a little push on the pins.
I think this may be a defective unit but I am not flash experienced enough to know how to check the pc signal to the flash.
Does Yongnuo have a good record of quality control or...not?
My current frustration tells me to return this to B&H and go to Ebay for a used manual flash.
Any advice from APUG users will be greatly appreciated.

I just tested my Yongnuo YN 560 IV on my Nikon F2 to see if I could find problems similar to yours. Finding a working PC to PC sync cord in my bag of old sync cords was harder than I thought. However, I finally found one that worked perfectly.

Therefore, if you are sure your sync cord is working, I would check to see if the Yongnuo flash mode is set on Manual Mode and not Slave Mode.

Another apug member explained how to check the circuit to fire the flash. The flash continued to fail. I think the unit is defective and I will contact B&H today for advice on how to proceed.
Thanks for your suggestions.

Please post so others will know how to check the circuit to fire the flash.

The member suggested by PM that I bend a paperclip into a "U" and touch one end to the center of the PC on the flash and the other to the cylinder to complete the circuit and "flash" the unit.