[Don Ro]

Did it rain or did you water the area any time after spraying Round-Up?
R/U takes a good 48 hrs. to translocate to the root structure - you may already know that.
Much quicker kill in warm weather/hot sun.

You could add some surfactant (several drops of liquid soap) into your mix as a sticker agent.
But R/U does have some already.

A pre-emergent weed agent will stop new seedlings from germinating.
Ronstar G is a broad spectrum granular agent sold to the commercial industry.
Put it down as per label (salt 'n pepper) and water in immediately.
It's used on grass as well, so no fear for your turf.
It's the best, IMO. I have two 40# bags in my garage - I use it all the time...large lot, ya know.

What are you doing wrong?

Maybe you're R/U kill is good but seedlings are still pushing - ya think?
In that case, just keep spraying them until no more seedlings, OR get some pre-emergent.

HTHs