Scratch

I'm trying to make a maze-type game. The idea is that the background is a group of squares, each containing a symbol. A sprite travels among the squares, but it has to only touch certain symbols. If it touches the wrong one, it has to start back at the beginning. I made a sprite that is the grid of the wrong squares. When my traveling sprite "touches" the grid, the level resets. Which works perfectly until I hide the grid. Then the traveling sprite goes right through the grid as if it wasn't there. According to this thread: http://scratch.mit.edu/forums/viewtopic.php?id=28 that shouldn't happen - even when hidden, the grid should be able to touch the moving sprite. What am I doing wrong? The "touching" code is in the grid script, not the moving sprite's script.