Please bear in mind that RPGMaker splits some of the auto-tiling tiles (where terrains blend e.g. water to sand) to achieve correct blending. So if the tiles are 16x16 pixels, in some cases it will split them into 8x8 pixel tiles to achieve correct blending. This allows it to implement auto-tiling with only 12 tiles, whereas for a full combination, you need 16 tiles. You need to make the 4 extra tiles yourself so you’ll have all the combinations needed.