You don't have enough IVs
Plain and simple. Cracking WEP works by statistical analysis, and in order to do that reliably, you will need lots of data.
The aircrack-ng FAQ says:
How many IVs are required to crack WEP?
WEP cracking is not an exact science. The number of required IVs depends on the WEP key length, and it also depends on your luck. ...