If you have a big area with a lot of gaps and planks levitating you should probably pull it up and start over. You can use the caulk method for several gaps but to keep them from (slightly) levitating you’ll need to get construction grade adhesive underneath and set something real heavy on it and let it dry 24-48 hours.

It’s a colored siliconized caulk/grout, it’s flexible and still holding up great. Here’s a link to what I used Your friend has shared a link to a Home Depot product they think you would be interested in seeing. www.homedepot.com/p/Custom-Building-Products-Polyblend-115-Platinum-10-5-oz-Sanded-Ceramic-Tile-Caulk-PC11510S/204984846

24 hrs