I don't think Corbyn could ever grasp the importance of presentation and being "on message" - two things at which, for example, his Labour predecessor Blair excelled.
Corbyn should - and could - have dealt early on and decisively on this whole antisemitism thing. But by dithering and refusing to put his cards on the table until well into the 11th hour, he left voters with a confused and ambiguous message about his stance on the issue.
And, I might add, Corbyn made exactly the same type of mistake over Brexit. It's an open secret that Corbyn prefers the UK to leave the EU; he should either have made his personal position crystal clear early on (and he could still have left plenty of room for pledging to put his deal to a confirmatory referendum alongside the Remain option), or he should have declared early on that the Labour party position was for Remain. If he'd done that, then of course there'd still have been many within his party who were staunch Brexiteers and who might seek to undermine the official Labour Remain position, but he was inevitably going to have to face that sort of issue regardless; and by coming out for Remain, he'd have immediately set out a polar differentiation from the Conservative position, from the only party which stood a realistic change of gaining more seats than the Conservatives in the GE.
I genuinely think that if Labour had officially come out for Remain, this could have been at least a very close-run election. But now we'll never know.
Again I absolutely agree. (I hope your heart isn't too damaged by regular agreement.....