A Broken Solder Pad
This is a common problem, especially when re-doing wiring. A combination of heat and the physical stress of removing a wire can cause the copper pad to break away from the copper trace leaving a fine gap between the two. The best way to check this is to use a multimeter to check a gap between the pad and the destination of the connection. Follow the trace from where the pad connects to another visible connection point. These can usually be repaired by taking an ink eraser and rubbing down the trace right by the pad to expose base copper and then heating the existed solder and use the iron's tip to drag the molten solder out over the exposed copper of the trace to make the connection. Recheck with the multimeter to validate that you not have a connection from pad to destination of trace. If the gap is significant, use a piece of fine bared wire to bridge the gap and then solder a trace and a joint is a good choice.