To be clear, this is the same Labour party that informally announced their refusal to be led by Corbyn before he was even elected to the position? I wonder why he's been so unsuccessful.
No they didn't. Stop making stuff up. The idea the entire PLP has been dissected multiple times. The number of MPs who were against Corbyn from the very start has always been very small. The vast majority of party MPs were willing to give it a go.
After a year of his leadership, labour are now looking at:
1) historically bad polling at this stage in the cycle, where labour should by all accounts be doing much better
2) labour *voters* saying in record numbers they can't vote for Corbyn - 30% in some polls. Same feedback was being given on the doorstep during the EU referendum according to the volunteers and activists I know.
3) the Uk leaving the EU, with a party leader who seems happy with that and undermined the campaign to stay in. Just like Trident, the MPs view that as breaking the deal the party had with Corbyn,
Corbyn has been unsuccessful because he's just a really bad leader. Blair managed to do so much despite facing a decade of attacks and opposition from Brown that was far worse than anything Corbyn had to deal with. Hell, Ed had to deal with worse. Corbyn is simply not up to the job in any way, shape or form.