i think the problem is caused by a fundamental - and probably genuine - disagreement of over who MP's 'belong' to. Corbyn - in stark contrast to when he was a backbencher - seems to take the view that MP's belong to the party and the party belongs to the members. the PLP takes the view that MP's belong to the electorate of their constituancies (remarkably enough, a view Corbyn held before he became leader...) and should vote/argue in a way broadly consistant with the personal and party manifesto that those constituants voted for.
so, for example, the Labour candidate in my constituancy got 16,000 votes in the 2015 GE. she campaigned on the Milibandite manifesto, and apart from some local interest stuff never campaigned on or talked about anything far from the LP manifesto. she didn't win, but assuming she had she'd have become our MP based on a centerist, pro-NATO, pro-Trident manifesto. why should she vote in a different way to the the promises she made to - and were accepted by - the 18,000 or so constituants she'd have needed to win just because perhaps 500 of her constituants voted for Corbyn in the leadership election?
(500 based on Corbyns 313k votes in Sept 2016 divided by the 620 or so CLP's. if you want to go wild you could divide the 313k by the 220 Labour MP's, which comes out at around 1400. even then its a pitiful number compared to the number of people who voted for whatever manifesto - party or personal - the MP stood on in 2015, and very few of them could be described as Corbynesque).