I downloaded a swimable pond by LeeFish, and am using the Frillen Invisible tile included with the download to change the floor tiles on the water. All went well until these last three tiles. They just won't become invisible. I've tried every level, and every adjacent tile, and I can't get the invisible tile to place over them. Any help much appreciated. What is the trick to getting the invisible tile to catch?

Sometimes you have to try a different angle to get the tile to select. You can also try to catch them by changing an area of flooring in one go, rather than tile by tile.