A few things could have been the problem:

- 3 weeks is an awfully short period to let the weeds grow before spraying. That spraying probably did nothing.
- The grass may have been growing low when you sprayed the tall weeds last fall or their seed heads were ripe
-Grass spreads from rhizomes (roots). You nicely planted the whole area with the crab grass when you disked it up.

Spray it all down when the grass is actively growing again. Don' t let it set seed.
Disk it under when you' re sure it' s all dead. It should get better as you do repeated kills.

Dan O.