Test by chalking the Knife edge all the way around the hatch
and then close and dog it. Open the hatch
and check the gasket. It should show an unbroken chalk line
all the way around. And if not? Then you may have to do the following.
1. Check the edge that the gasket comes up against. Is it straight and square to the gasket? Have seen some knife edges bent, concaved, convexed and wavy. And there was no way that a good gasket could get a water tight seal on those edges.
2. To fix mis-alined edges you may have to file, hammer, adjust hinges
and/or dogs, or force that edge into alinement, rebed or replace the whole unit.
Chalk testing your gaskets quarterly/seasonally is good preventive maintenance. The chalk? From your kids' toy box or get one that is made for the task.