Boats are constructed with interior baffles or bulkheads that separate the chambers. When the pressure in one chamber drops, the adjacent chamber (in some cases, all the chambers) will go soft because the baffle/bulkhead migrates into the lower-pressure chamber, creating less pressure in the previously "full" chamber.

The only way to go about it, is to fix the leaks as you find them. If you seal the pinhole leak on the bow and the boat is still going soft, that means there is another leak somewhere (could be the seam, could be another pinhole, etc...)

Stephan