Vith makes a good recommendation that works. Here is what I have done and it works like a charm:
Vacuum out the debris from the gap.
Purchase a "glass holder" or suction cup (two suction cups connected with a handle - Harbor Freight $6.99).
Clean the plank and apply the suction cups an inch or two away from the gap (Remember - nothing sticks to dust). Securely attach the suction cups.
Apply a small bead of vinyl seam sealer or tongue and groove wood adhesive to the exposed tongue.
You may want to remove a piece of baseboard and insert a shim to hold to row in place and protect the expansion space provided for during installation.
Gently tap the plank into place with a rubber mallet, being careful not to over tap and damage the locking mechanism or the wear layer.
Wipe off the excess adhesive that will squeeze to the surface immediately with a damp cloth.
* If the gap you are working on closes up and re-appears at the next joint, repeat the same process until the entire row is free of gaps.
This is the most in-destructive way I have found to deal with the occasional gap in laminate and floating vinyl plank floors with locking systems. 100% success rate over 5 years (nearly 100 attempts of various laminates and vinyl floors).
Do me a favor and keep this little trick to yourself. No caulk, no filler, no removing large portions of flooring - 30 minutes tops in most cases - you can do it.
Keep in mind, this will not work if there is a fundamental flaw in the installation or the locking system is damaged - you would have to insert a new plank(s) before beginning this type of repair.
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