I was selling my house and had a few spots where the planks were coming up from the slab. I drilled a few 1/16" inch holes in the areas and injected some gorilla glue (the kind that expands as it dries) inside with a plastic syringe. I put some weights on top while it dried then cleaned it up and stained it so you couldn't even see the holes unless you looked closely. Seemed very solid afterwards.