We just installed a shower in our basement. We used marble 12" tile for the walls and marble 1" tiles for the floor. We did a flood test by plugging the drain and filling the pan with water. It held and did not have leaks! The problem is when we open the drain and run the shower. The water slowly moves into the laundry room floor. COuld our drain be too high, or cracked? We installed the tile right onto the concrete floor in the basement.

So this would mean you did not create a shower pan with sloped sides, right? Was there already a drain in that concrete floor or what did you do to make a drain? It does sound like your drain pipe is the culprit at present.