Re: why Corbyn / Labour are polling so badly atm: there's a few reasons.
Corbyn faces a totally unprecedented smear campaign against him, that takes in the entirety of the mass media and many of Labour's own MPs and Labour-aligned commentators, advisors and allies. I think Tony Blair explained the Labour right wing's position best during the infamous hear transplant speech, 'I wouldn’t want to win on an old fashioned leftist platform. Even if I thought it was the route to victory, I wouldn’t take it.' - they would rather another Tory victory in 2020 than put their shoulders to the wheel and work towards a Labour victory under Corbyn, such is their ideological opposition to his policies. Obviously, they're hoping his position will become untenable before then: but if not, then the right wing will be ready post-defeat to take back the party. They have no plans that involve Corbyn taking Labour to victory.
So, while Corbyn has made significant attempts to draw together a team from all wings of the party (making a virtue of necessity, as his own wing - in the PLP at least - is so tiny), the right wing feel no compunction to make any significant concessions as they've no intention of him staying in place any longer than they can help. This results in something that should be a fairly simple job - say a minor reshuffle - taking several days, as complex negotiations need to be carried out with every minute change to prevent the shadow cabinet collapsing altogether.
The drawn out negotiations can then be spun by their allies in the press as a shambles, another example of Corbyn's incompetence, as well as another example of how badly divided the PLP is.
These are the biggest issues - that the main medium by which the politician reaches the public is so relentlessly hostile, and that he is trying to fettle a parliamentary group that is actively attempting to sabotage his chances of success. I don't see either of these things changing in the near future.
There are other issues: the lack of experience of the leadership team has led them to making some fairly elementary mistakes, which have - of course - been magnified a hundredfold by their enemies both within and without the party. There's also a dearth of talent on the left of the party (there's only about 7 of them after all) meaning that walking disasters like Abbot and Livingston end up being the visible face of the leadership. And the many of the politics they push, while not being particularly radical, are far enough away from the current political mainstream to be characterised as such and dismissed by the mainstream. But none of these are fatal - the main two may be.