Crabgrass preventer simply prevents seeds from germinating so naturally ir isn’t going to have any effect on weeds that are growing from an already established roots system. In addition, if you applied your crabgrass preventer two weeks ago or longer, it probably isn’t going to even prevent the crabgrass. You need to watch the forsythia bushes and wait until they start dropping their yellow flowers. This is the time when soil temps are just right for crabgrass seeds to start sprouting and this is when crabgrass preventer prevents that. Now (5/14/20) is just the perfect time. Crabgrass preventer is a delicate thin barrier on the soil and it breaks down quickly so it is best to wait for the correct time to apply it based on the shrubs/soil temperature. Some years is would have been 3 weeks ago…this year not until now.
It also isn’t surprising that youe weed killer isn’t working since you need NIGHT TIME temperatures to be consistently 50° or above for it to work. At low temperatures, the weeds aren’t growing quickly enough to absorb the weed killer. Again, we are only now getting to the point, almost, where temperatures are warm enough for weed killer to work well. For clover, you’ll need a stronger lawn weed killer like Bonide Chickweed and Clover Killer. Remember, weed killer is absorbed through the foliage of the weeds so don’t put it on if rain is in the forecast within 48 hours. If the weed killer is washed off the leaves, it won’t work. https://www.bonide.com/products/weed-control/view/061/chickweed-clover-and-oxalis-killer-conc