Well not sure what your problem maybe but it sounds like there is no network activity to give you anything to ARP. I always us aireplay to crack WEP keys. Seems to work for me. I mean you have to make sure someone is going to be using the network is some way. Here are the two lines I use while using airodump
aireplay-ng -1 0 -a (bssid) -h (your mac) (your wifi card)
once that finishes
aireplay-ng -3 -b (bssid) -h (your mac) (your wifi card)
once this is running in a new shell as well as your airodump command, and someone hits that network with anything at all you should be able to watch your data packets go crazy should shoot to about 10,000 in less then 5 min with a strong signal. Once done then you can just use aircrack to crack the key. Hope this helps.