Simply put, bugs that need to be worked out. Failing resets the combo, but that's not how it was explained. Line gems don't always go through all the obstacles/gems in it's path. The randomizer isn't so random. It happens more often than not that when you need a certain gem it won't appear, at all anywhere near where you need it, no matter how many gems you match from that side. The new gems are always all the other gems that you don't need.