"Soft deck syndrome" most often results from heel dents/dings that indicate a board has been sailed very hard, or jumped and landed flat and the impact of the sailors heel has compressed the foam layer under the deck.
It can be fixed, but I do not think putting one hole in the dent and injecting foam will work as the foam needs to spread and expand. putting the foam in a central hole, with
several smaller holes around the periphery of the dent might work.
The best way to fix it is to open up the entire dent, remove the crushed foam, then add
the layers of foam and glass back in the same "build up" used to manufacture the board.
Eva takes you through all the steps to correctly repair "soft deck".
Hope this helps,