Math.Random Producing Repeating Results

I've added Math.RandomSeed(os.time()); to my On Startup code but whenever I call Math.Random(6, 12), it always returns 12.
I've looked on the forum and in the manual but did not see a solution. Also, I've tried this without seeding since the manual states that the app is seeded on startup; however, I get the same, repeating results. Why might this be occurring?