The synchro aka. PC jacks of the Neewer S-400N flash are total crap.
The slightest vibration causes plugs to literally jump out of the jack.
The reason for this is that the barrel of the jack is a bit (about 0.5mm) too deep. This prevents the flat spring (that ~5mm wide metal piece) shown here to latch onto the tip of the plug.
On the right side, you can see the back of the jack with the plug inserted barely catch the tip of the plug.
On the left side in comparison a jack whose barrel I shortened by filing it down about 0.5mm. Now plug can be inserted a bit further into the jack. Enabling the spring to latch onto the tip of the plug (the arrow points to the contact point of the flat spring with the tip of the plug)
On the left the modified (shortend) jack, on the right the original on.