Ev - The reason that this happens is because laser light is only one color of light, and light from your flashlight is all different colors of light. This probably sounds pretty weird, but let me explain. There are lots of different colors of light. Just like with different colors of paint, you can mix them together to get other colors. It's just that light colors mix differently than paint colors.
Prisms bend different colors different amounts.
So when you shine a flashlight in, all of the different colors bend differently and get separated. Lasers, on the other hand, are only one color. Some of them are only red and some of them are only green, but they're only that color. So when you shine a laser through a prism, there's nothing to be separated, and the light stays together. One thing that I think is pretty interesting to see is if you shine two different colors of lasers through a prism.