A WEP key can be either ASCII or Hexadecimal and some network adapters don't like an ASCII key. You can tell by the number of characters which type you're using:
64-bit WEP key uses 5 ASCII or 10 Hex characters.
128-bit WEP uses 13 ASCII or 26 Hex characters.
(ASCII characters are basically the letters, numbers & symbols on the keyboard. Hexadecimal characters are the numbers "0" to "9" and letters "a" to "f" only, a total of 16 available characters.)