I have a recurring problem with wild violets in my back yard.
As other alluded, the leaves are rather waxy and shed herbicides rather well.
To make your spray stick to the foliage better, add a squirt of dish soap in your sprayer. You can do this whether you are using an Ortho dial-n-spray or a tank sprayer.
2-4D Amine requires a minimum of 5 hours dry period to attain a systemic effect on the plant, but ideally you should shoot for a 2-3 day period of no rain after application.
Wild violets are especially tough to eradicate since they produce a root system with runners or rhizones, so repeated application may be necessary. I would come back in a month or so and hit it again if necessary.