Corbyn is strong when he gets the chance to talk about policy which is where his passion lies. He's a great activist politician, just not a great leading one. I've always liked the man and many of his policies, I just think they won't chime with the electorate or are a good idea but not thought through with care. And the Labour Party is in such disunity at the moment that I wouldn't trust them to renationalise the Parliament cleaners, let alone the £66bn water industry. Although if Corbyn did turn it round and win then it would be a seismic shift that nobody could fail to back - it could change British political landscape for generations. I'm just not confident that he can do that without appealing to the aspirational middle classes, and I don't see many aspirational middle class policies there.
There's a problem with his election strategy also. He seems to be visiting mostly echo chambers - Manchester, Leeds, Liverpool; they already are going to vote Labour. What he isn't doing is attacking marginals which can win him seats, which you'd imagine he'd want to be doing. There's a rumour in the Party that he's trying to up his popular vote numbers in order to show that his policies are popular which will give us either a post election Corbyn (doubtful) or a Corbyn ally (possible) as next leader. Energising current Labour voters to turnout at the polls rather than winning Labour any new voters.