I have a problem that I've been embarrassingly working on for months and unable to completely solve. I'm a hobbyist and I have a simple PCB with a male USB Type A connector soldered to it: some of my boards work ok and some only see the power LED light up when I jiggle the connector around (boards are from the same batch). At this point, I am nearly positive it's a mechanical and not a circuit problem, but I cannot figure out what I am doing wrong and am seeking suggestions. I've tried different female ends (i.e., different cables, USB hubs, etc). I've tried different kinds of male Type A connectors soldered to the board (surface mount, through-hole, etc), with the through-hole connectors being the least problematic, but still some of my through-hole-type-A boards exhibit the problem (some don't).
Could it be rosin solder flux residue on the Type A contacts that result from me trying to clean off rosin solder flux on the PCB using a rubbing alcohol bath? The boards look clean but I'm wondering if just enough residue is being left on the USB contacts within the connector to be causing me problems. It seems far fetched, but other than this, I can't think of what else it could be -- I'm about out of ideas.
EDIT: When the problem exhibits itself, I see 0V when I put a voltmeter between the 5V and GND pins on the Type A connector.