Usually herbicides work best when weeds are "actively growing". What they mean by that is after a period of rain or irrigation. During times of drought stress plants close up shop, so to speak, the stomata &c. Consequently it's best to apply after a good rain or irrigating for a couple days. It's much more effective that way. That might be what's going on here.

April is a good time to spray for summer weeds. The fall(ish) rainstorms are when the winter and spring weeds sprout and then go dormant. So as mentioned, wait for the rain and then spray in the fall.

I almost always spray twice. Fall is the best time for broadleaf weeds inmho. Even though topgrowth may not change as fast, rest assured they are moving herbicide to the roots.