Have you tried a fragmentation attack, and then inject the collected packet to collect IV's? Also, why don't you use Gerix-Wifi-Cracker that already comes with BT4 tools suite? It's more than complete to use in WEP attacks, and pretty much user friendly. You can start with frag. attack, once the packet is ready, start ARP request? (not necessary), then inject the packet until you have enough IV's.
I've heard somewhere some new routers come protected against wep injection, but i'm not even sure this is true..