Atrazine is usually a slow-working herbicide, not a fast burn-down treatment. On emerged susceptible weeds, visible injury often takes about 14 to 21 days to become obvious, and full control can take longer depending on weed size, weather, and whether the product is being used for post-emergence or pre-emergence activity. In other words, if you are expecting same-week browning across the whole area, atrazine will often feel slower than you expect.