Try a different herbicide and apply it according to the label, rather than using one that doesn't work in an off-label manner. Sandy soil is permeable and doesn't have organic matter or clay to bind/react with the herbicide, it is important to use herbicide according to label to avoid ground water contamination.

The label does not say anything about the above. I am applying it according with the label (weed b gon)

Violets are pretty much untouched by herbicides. I dig mine out.

There would be no point using herbicide on violets. They seed proliffically and also have runners. Best to keep pulling them out to, over time, eradicate them. A bit of a project but in my experience, we get more done than we think we will manage if we chip away at it.

A digging tool such as a hori hori knife might do a better job than your fingers, be easier, and not limit you to after ain.

They never totally go away just keep pulling and it keeps them from running over.

Keep digging because they will seed (cleistogamous) all year. Needs constant vigilance...but the roots are neither deep nor wide and can be removed with relative ease. I second the hori-hori.