Elle,
You've gotten good advice so far. Seems like things are pointing to the main drain. When I was looking for my leak last year I taped a piece of small diameter auto vacuum hose to a 10' PVC pipe, 1/2", and used that to squirt dye at the deep end. Used a 1 gal. garden sprayer for the dye. Water has to be very still and you need to squirt just a little bit and watch. 3" a day in my pool was about 3 quarts per minute which is easy to find. For dye you can get a dye marker at a dive shop, or boil chopped beets.
Al