poor connections between the male and female terminal at a connector, a terminal not seated all the way into the connector body, and poor terminal to wire connection, such as poor crimps and corrosion.
on to the ground screw terminal at each electrical receptacle, and ending with a ground clamp crimp connector that ties the incoming ground to the ground wire of the outgoing wire that continues to the next junction box. If the incoming ground wire is not long enough to run as above, then...