The Labour right are self perpetuating apocalyptic nay sayers. They act in every way possible to undermine the democratically elected party leader, do sod all to canvass or aid the LP, unless it's in a safe seat for a Blairite, and then dive in like vultures if there is a poor result for the party.
It is no real mystery why the Labour Party is doing so badly north of the border. It is a lethal mix of organisational incompetence married to political opportunism.
There is a huge space to the left of the SNP. The SNP are not left wing in any meaningful degree, and could easily be out flanked to the left, but Labour can't do this. The reasons why? Decades of Tammany Hall corruption have meant that local Labour Parties are simply personality cults/electoral machines for whatever sectarian variant wants to be elected. The Labour party, at council and national level, has been complicit with cut-backs and austerity programmes with barely a murmur raised in progress.
Personally, and politically, I admire Corbyn and the Momentum movement but they are too little and too late to save the party north of the border, and being far too mealy mouthed in the south. The Blairites are now in open rebellion and doing everything in their power to a/ replace Corbyn and b/ win the next election for the Tories.
The left in the party must fight back with everything they have, and they have a lot, namely the huge advantage of an overwhelming mandate.