Some of that is his own doing. You can't really spend your entire leadership treating the press as the enemy, being largely unresponsive to requests for interviews etc, and then when election time comes expect positive coverage. You need a media strategy to woo elements of the media who can help get your message across in times like this. That's part of what being a leader should be able, being a good communicator. Until the past couple of weeks there's never seemed to be much appetite to talk to anyone unless they already agree.
Being poor in that sense as leader can be manageable if you've a team around you who handle the media well, but Corbyn's inner circle seems to view the media with hostility and mistrust too. Perhaps understandably but if you're in that position you need to accept the media is arguably the best way, still, of reaching a wider audience in a way that Tweets and attending rallies held in your own honour isn't going to.