There are pieces of foam floatation inside the hull that could be preventing the water from getting to the drain. There are two options. Sometimes the water moves more easily fore and aft. Try tilting the boat so that the bow is higher than the stern - the more tilt the better. If the water runs aft (you'll hear it) then the best approach would be to drill a drain in the transom into which you can put a removable/screw-in drain plug.
If that does not work then it may be necessary to install one or more 4" inspection ports so that you can look inside the hull to see what's happening.
Before you do any drilling, it would be a good idea to find where the boat is leaking and fix that. Mix some soapy water and sponge/brush the deck (especially where hardware is attached), hull, hull/deck joint, and centerboard trunk. Then use a shop vac, hairdryer, tire pump or some air source to blow air into the drain hole while someone examines the boat looking for bubbles. Where air comes out, water is getting in.