The fix involves applying a tiny bit of wood glue in the open tongue and groove gap, then sliding the flooring together with your hand. I find a toothpick works well for getting the glue down where it needs to be on the exposed tongue itself. You could use regular white or yellow wood glue, but I prefer Chair Doctor (www.leevalley.com; 800-267-8767). It has very low viscosity that allows it to wick deep into the joint.
Before you apply glue, however, make a dry run, experimenting to discover the best way to move the flooring and close the gap. I find a sharp, angled blow from the palm of my hand works best. As soon as you’ve glued and closed the gap, wipe off excess glue with a damp rag. You should also take a look at other boards along the same row. Sometimes closing one gap opens another, but that’s okay. Simply glue and close new gaps the same way.