Crabgrass preventer will only prevent crabgrass from germinating not kill crabgrass. First application goes down when the forsythia start turning yellow. If it rains heavily after you apply it it can run right off. If the crabrass is already actively growing it's very difficult to kill. Usually wait for cold weather to kill it off then prevent it. I usually get stuff from Home Depot or Lowes. This. https://www.homedepot.com/p/Spectracide-10-8-lbs-Weed-Stop-For-Lawns-Plus-Crabgrass-Preventer-Granules-Up-To-5-Months-Of-Control-HG-75832-2/202559024?ITC=AUC-81109-23-12140
Weed (dandelion, clover, chick weed) control only works on weeds not crabgrass. It kills when in contact. If the weeds are already there you need to kill them out first then rely on preventer as the year goes on.