deck delamination
this is usually an indication that the core is rotted out. I have an aversion to injecting epoxy into what is probably a mess of rotten wood, moisture, fungus, bacteria, you name it. Another option is carefully use a zipit tool or equivalent and cut out a whole panel of deck , dig out the bad core, replace the core, epoxy the core to make it waterproof, then bond the deck panel back in place, you will have a small seam all around to try to tmatch the gelcoat.