However, I can't seem to sweep this particular private key (Electrum says no input found),
You may have to add the script type prefix in Electrum when sweeping your Bitcoins.
Just click the tooltip button (
Info) in 'Sweep private keys' window to know which prefix to use depending on your address' script type.
But if your address is
P2SH-P2PK, Electrum wont be able to sweep it.