When I would fire the cannon, its 'shell' would explode immediately as though it was hitting something up close.. The only time it would not explode was if I would fire it at a very high angle. I thought at first maybe it wasn't on level terrain, or maybe there was something on the terrain that blocking it, but didn't see anything.. I tried a few different things such as moving it around and selecting different terrain types with no luck. But once I unchecked the 'Can Collide' option it started working correctly..