Labour has an almost impossible problem, caused partly by the referendum. It's losing its Leave voters (mainly older traditional people) to Ukip and its younger Remain voters are going back to the LibDems. It doesn't have a clear message about Brexit, so rather than keeping people, it's losing them.
It needs to have a much stronger narrative on the NHS and other issues, which trumps people's views on Brexit. Corbyn and his supporters don't want anything to do with any agreement with the LibDems, whom they see as right wing traitors.
It was a LibDem amendment in the HoL, which resulted in the government's defeat over EU citizens. Labour peers voted against a LD amendment to keep the UK in the single market. It's a confusing message. Many of the younger (and other) people who joined Momentum aren't impressed.
My personal view is that it's too late for opposition parties to work together and we're going to see a major realignment of political parties at some time in the not too distant future. Labour is already a split party.