I went to route 211 in Sinnoh to catch a Tyrogue with my poke'radar, but it won't Let me. (Rowans words echo... blah blah blah) I'm in grass, and Dawn already walked me through the steps.
It may be either:
It's not fully charged.
You may have the wrong item registered if you're using Y(button).
Make sure you're in regular grass and not the high kind ( the kind that prevents bicycle travel) <<<this one got me :p
Hope I helped :)