When was the field mowed after spraying? If it was after three days, then the plants that were treated will die off. You probably should plan to retreat the areas that weren't killed after those plants are growing again. (It will take a few days depending on moisture and other conditions.)
In general, you can spray any missed plants 7 days after the first treatment. Then mow or seed the area 3 days after the second treatment.
If the field was mowed prior to the three days after the first treatment, retreat the entire area after the new growth begins.
For specific details, read the label for your Roundup product. Scotts has the label at the bottom of their product pages. All of the Scotts Roundup products are listed here:
http://www.scotts.com/smg/catalog/productCategorySubSelf.jsp;jsessionid=64F09279DC15FE8877450FE65C85C38E?itemId=cat50096&navAction=jump . Select the specific one you used and click on the View Product details link. Then click on the View Label link. For example the label for the Roundup RTU (ready to use) Pump and Go Spray formulation is here:
http://www.scotts.com/smg/products/round_up/kill_and_prevent_weeds/pdf/BasePNG.pdf .
I hope this helps.
Cindy Wells