In the electrum console on the offline wallet try to run this command a few times. It will generate a new change address so you'll have the private key in your wallet and be able to sign the transaction:

wallet.accounts[0].create_new_address(1)

To generate new main account addresses (in case those are the ones that are missing) do this:

wallet.accounts[0].create_new_address(0)

Or increase the gap limit. Say 20:

wallet.storage.put( 'gap_limit', 20 )

You may have to restart the client after you do this.

Source for all of this is the developer himself:

https://bitcointalk.org/index.php?topic=324614.msg3604327#msg3604327