https://electrum.orain.org/wiki/Frequently_Asked_Questions#Can_I_import_private_keys_from_other_Bitcoin_clients.3F
Agree, I've also had this problem before on an earlier version of Electrum. I was however able to import it into my Blockchain wallet so did that and just moved the funds back to Electrum.
Was most probably something related to the format of the private key.
I highly recommend not to import private keys to Blockchain.info or other online wallets, if you are going to use that addresses. Anyway, as you can import private keys to Electrum, I hope you won't import it to an online wallet.
this, thanks!