Grignard reaction on electron-poor benzyl bromides
This question has aged a bit with no other answers, so here I go.

The same nitro group that can oxidize Grignard reagents could be oxidizing the magnesium too, thus keeping it passivated.  What you need is not so much a milder EWG but one with less oxidizing potential.  Try your reaction with nonoxidizing EWGs like acetyl (watch for side reactions), a $\ce{CF3}$ group, or a chlorine atom attached to the aryl group.