From time to time, I have had quads simply "fall out of the sky" for no good reason.
Today, I took a small quad out and armed it. When I gave it a little throttle, all the motors started spinning - except the right rear one.
It just jerked a bit. I dis-armed the craft and armed it again. Same thing.
I looked at all the connections. They were tight, but I knew something was not right. If one of the two wires that feeds battery power to the ESCs was loose, the motor wouldn't spin at all, but if one of the three wires that went between the ESC and the motor were loose, it would jerk - just as I had observed.
I got out my ohm-meter and did some checking. I had earlier fashioned some special probes for my voltmeter that consisted of two MOLEX style female crimp pins that fit over the meter probes, but instead of crimping wires into "wire end" of the MOLEX pins, I crimped (and soldered) two medium-sized sewing needles. The needles allow me to probe very small parts, and also to puncture insulation without damaging it.
Two of the 3 wires between the ESC and the motor had very good connection. The third was flakey - I moved the wire back and forth and sometimes I got good connection, sometimes not. The problem seemed to be the 3.5mm bullet-style connectors themselves. But they were tight. I inspected them carefully and noticed one thing: The expandable parts of the male connector freely rotated on the rest of the connector. The connection between the expandible part and where the wire was soldered was not a good electrical connection. I replaced the connector with what is normally considered to be a more "crude" type and got a great connection.
A picture of the BAD type is shown below
Note that the expandable part is a separate piece that is put onto the connector. Then the end of the connector is "mushroomed" out to hold it in place. The key item is that the expandable part has no guaranteed connection to the rest of the connector.
A picture of the GOOD type is shown here:
This connector is one piece. the 4 "fingers" are attached to the base and do not rotate.