One, are you sure your damage is from grubs? Many diseases will kill grass. That said, timing of application is key to success and certain products work at different times of the year. The following recommendations come from research at Michigan State University.
Scott Grub-Ex is best applied between April 15th and May 15.
Bonide Grub Beater works best when applied between June 1 and July 15.
Bayer Advanced 24 Hour Grub Killer works in fall
If the labels tells you to water it in. Irrigate the area with 1/2 inch of water. Use a tuna can or a cat food can to catch the water for 20 minutes and do the math.
Hope this helps
Nancy and Happy Yardening