Water hemp will produce a new flush of weeds with every rain.
My corn fields were perfectly clean at waist high. They still are in the middle were the ground was shaded out but around the edges there is hemp from 2 inches tall to 2 foot tall.
If you could have sprayed again a couple of weeks after your first pass you may have kept it under control.
Or wait till later next year when the grass is taller to shade it out.
About the middle of August it will give up for this year and come back again next year.
If you can control it without it going to seed for 5 years or so you may not have to fight it as hard after that.