i believe those private keys are in a special shorter form called mini private key format that can be found here:
https://en.bitcoin.it/wiki/Mini_private_key_formatin that case the key has to be converted to a normal WIF first then that should be imported in a wallet. casascius has some tools on github for it.
Electrum supports mini private key. Casascius's mini keys were 22 characters instead of the usual 30. Electrum will support anything more than 20 if it is valid.