guys i have problem, here, because i don't know what i'm doing wrong.

i boot the cd, start bt4 and insert all he data that we need to desencrypt a wep wireless lan, an in thefinal step, when aircrack is trying to find the key, it show's to m a message that says:" Failed. Next try with 5000IVs."

can anyone tell me what does this mean, and ewhat can i do to get the key...