The main active ingredient in Roundup is Glyphosate isoproplymine salt...lots of weed killers out there have GIS as the active ingred. But since you already have R/U...
Be mindful of the American approach - More is better.
You don't want to over-mix it - adding more than the label instructs you to do.
If so, you risk the GIS burning the tops and killing the vascular tissues in the above-ground parts of the plants (top kill). The plant may look like it's dying, but in fact, the quickly-dead above-ground tissues are no longer able to translocate the killer agent down into the roots where weed kill must occur.
Good luck...be patient.