One mistake made a lot of times with replacing the seal between the water tank and the bowl is that some people tighten the bolts too tight which can actually distort the gasket. YEP ! The bolts between the tank and bowl should have the rubber washers under the bolt heads, the metal washer should go up under the bowl. The hole for the bolt should be a bit larger than the bolt and the washer will allow the nut to have more seating surface, I like to add a couple of extra metal washers so that they do not bend. Turn out the light in the bath/toilet area, use a flashlight to view between the tank and bowl while flushing the toilet. Look at both sides carefully. IF this is where the leak is (and I'm betting it is) you will see the water run out this locating the problem.