As I keep saying to the Clownbynian supporters - it isnt, really, the policy that is the issue but the man. He isnt up to it. He never has been and the public regard him as a joke.
Yes. He comes over as clumsy, unintelligent, and inflexibly latched on to a particular set of beliefs.
However I think things run a little deeper than that.
For a start he is surrounded by people who are inexperienced, and in many cases deeply unattractive even to elements of his core vote. For example Diane Abbot may have some appeal in London, but she doesn't offer much to regions like the north-east or South Yorkshire, let alone more marginal areas. Meanwhile he struggles even to fill his shadow cabinet.
The party is badly divided. The Conservatives were bitterly divided over Europe in the 1990s, and this was a strong factor in their historic 1997 defeat. Labour's divisions now are more fundamental as the party is divided over its core direction, plus some pretty important issues like defence. No one seems to know what Labour's position is on Europe, is Corbyn pro Brexit or not? No one really knows.
The policies may not be too bad, but because the presentation is so appallingly bad, the general population don't really have a clue what the policies actually are. It doesn't help that most of the shadow cabinet are names no one has heard of, and in any case they keep changing on a revolving basis.
Every election is won in the centre ground, this is why Labour lost so badly in the 1980s, and the Conservatives lost so badly in 2001. Tony Blair and David Cameron, love them or hate them, understood this very well, and I strongly suspect Theresa May does too. People may sympathise with elements of Corbyn's politics, but when it comes to the privacy of the polling booth people will be thinking of things like the economy, defence, perhaps immigration, competence to govern, taxation, et cetera, and in all these areas Corbyn is not in the centre ground. If anything the centre ground has shifted slightly to the right in recent years, which derailed even the reasonably centre-ist Gordon Brown, and meant Ed Miliband lost badly. Unless something else changes, Labour has been heading in precisely the wrong direction -- with Gordon Brown and Ed Miliband being rejected by the electorate, the worst thing to do was to pick someone more to the left.
The problem is, who replaces him? There's no obvious candidate who appears electorally viable, and the likelihood is a candidate with similar views would be chosen. The fact that the prospect of Tony Blair or David Miliband returning has actually been seriously suggested shows how bad things are for Labour. Things go full circle however, so I wouldn't necessarily write Labour off, but they are in big trouble.