Distance: if your access point is upstairs and you’re testing it from downstairs you may find you’re unable to authenticate with it to generate traffic – move upstairs instead, otherwise you may get a false sense of security (it may be that someone with a directional aerial could be more successful in breaking in than you were: we’re using a cheap network card remember?).
Parameters: get the parameters to airodump and aireplay right – if you get them mixed up, you may find that you fail to generate traffic where someone else might suceed.
Aircrack failure: take care with this one – if you increase the fudge factor and the number of packets captured, you’re likely to find that you can break a network that might initially seem to be immune, just because you were ‘unlucky’ with the packets you captured. Don’t give up until you’ve captured a couple of million packets and doubled the fudge factor a couple of times.
Aircrack can helpfully fail due to a dynamic WEP key – this just means that your network is more secure than it would otherwise be with WEP, and demonstrates the relative strength of this technology vs. static WEP. If you’re finding it tough to get in, it’s likely anyone else will too.