Solder/connection issue
often some flux may migrate beyond the connection site, wicking into connector bodys. especially small, confined types used on models and other delicate equipment. while the electrical circuit may be complete between the wire and the connector, the actual connection between male/female parts of the connector may be fouled by the flux. it may be necessary to use isopropyl alcohol (90 percent) to wash/flush/dissolve the connector free of the flux. despite the wiping/tightness between metal contact parts, flux becomes very rigid and acts similar to shellac when it becomes cool at room temp, acting as an insulator between the parts.