One of the engines does not spin

My problem is this:
I have an optacopter in x coaxial, the flight controller is pixhawk 2.1.
Everything seems to work fine except one of the engines.
When performing the test of motors all work except motor “E” that corresponds to the output of pixhawk 2.1 to number “3”.
The ESC and motor work perfectly and the ESCs are calibrated, I have tried it in another arm and it works perfectly.
I have tested with fimrware V3.5.3 and V3.4.6 the problem is not resolved.
I looked inside the pixhawk2.1 in case some connection was badly welded … everything seems to be in order.
I do not know what else to look for to solve it.
I would be very grateful if anyone can help me.
Best regards

Last test before sending the Pixhawk back for replacement is to move one of the other motors onto the #3 channel and see if it works.If no it’s RMA time.Mine is back there being tested as I type.

Thanks for your answer.
I already did the test you mentioned and # 3 still does not work.