Jammed?
In normal operation the plexiglass hatch cover would slide forward until the plexiglass clears the cover strips on the sides and where the wood lateral brace at the aft end of the cover comes against the fiberglass hatch cover. There must be something blocking full forward movement of the slider, or it is binding on the side tracks. Reaching up from the inside with a long finer and feeling inside the drain channel, you may discover the problem. Or, from inside, using a Phillips screwdriver, remove the wood cross stop that is screwed to the forward end, top side of the plexiglass cover. This should allow you to pull the sliding hatch cover toward the stern of the boat. With it completely removed, the fiberglass cover can be raised to inspect the drain channel and other parts of the sliding mechanism to discover the problem. You may find that the drain channel is filled with debris. Also, you may find that the bottom side of the plexiglass cover where it slides on the side rails has accumulated a layer of gellcoat from repeated sliding on the rails. Use fine steel wood to remove the buildup, and give the edges of the plexiglass, as well as the fiberglass rails, a good coat of wax. Hope this solves the problem.
