This is a guess, but it sounds like you have Johnsongrass. My response is for that problem. If it isn't Johnsongrass, there may be another solution.
It is a perennial weed, but also comes from the (annual) seed it produces. Non-selective weed killing products can kill the top, but you've got to get it before it goes to seed. It has very deep roots which stalls weed killers from completely eradicating it. Seeds that have been produced may last up to 6-10 years of viability in the soil!
The best answer is to use glyphosate on known Johnsongrass areas and to mow, mow, mow to prevent more seed production.
It may take persistance and years.
Hortster

Yes, it is Johnson grass. I googled it and Google gave an exact description of it. I am sure glad I didn't have my neighbor rototill my garden. Every stolen would have turned into a new plant. I see years of trouble.

The only way to get rid of Johnson Grass is to dig out the roots, because as you have seen the spray commonly recommmended for it does not control it very long. Every spring, summer, and fall I dig out both Quack Grass and Johnson Grass roots from some of my planting beds, often pulling a root 6 feet long out of the soft soil in those beds. There just is no other method of control.