Why do thistles keep growing back, even after using Round-Up?

Question:I have removed my flowers from the flower bed, used Round-Up to kill everthing else off, then used the tiller. Raked out the dirt, fertilized the soil. AND the thistles keep coming back. A ton of them!! How do I kill them? Why can I do to the soil to keep them out?

Answers:
There must be seeds in the ground that are germinating. Round-Up doesn't kill seeds. Just keep spraying and spraying.

Or cover w/ a few sheets of newspaper and more mulch, and choke them out.