Back when the EU referendum was gathering pace, there was an odd pattern that kept cropping up. Jeremy Corbyn would declare that Labour was backing Remain. Then, in that article, there would usually be a short paragraph that said something along the lines of 'doubts remain about Corbyn's true loyalties'. The commentators, invariably the same thing promoting the line that Corbyn was unreliable on Europe, would then express their shock and outrage when polls dutifully informed them that people were 'confused' as to what Labour's position actually was .
This is a prelude to the current discussion, namely the perennial question of whether Labour should be anywhere between 10-20 points in front of the Tories. The rough argument is that if Labour backed being members of the single market they'd be romping home. Naturally, whenever this is queried further as to how, why and where they'd get the voters from, the person saying it suddenly remembers an urgent appointment elsewhere.
A familiar point to make is that the right-wing press are part of this, with their full throated backing of the Tories. A counter-point is that this should be taken as read and Labour should attempt to work around it. I agree in theory with this, but the focus on the right-wing press obscures the dynamics. The real problem is that the main outlets are the right-wing press and the centrist press, the latter of which regulalry concentrates on attacking Corbyn (as the representative of all that is bad about the Labour party and its Brexit policy). The centrist commentators have largely managed to convince themselves that (1) Corbyn has the power to stop Brexit; (2) but he won't because he wants a hard Brexit . There is, thus, very little out there that's giving a realistic portrait of Labour's Brexit policy and why it is the way it is (Stephen Bush is the one that comes to mind).
The reasons for this are the same as the reasons why there was a need to obscure Corbyn's actual support for Remain: because staying in the EU/stopping Brexit is second priority to regaining control of the party, or helping their mates regain control of the party . That was one of the key factors that scuppered Remain, and it's one of the factors scuppering a push for a soft Brexit (i.e. the Tory rebels know that they don't have to actually carry through with their rebellions, because they'll still receive praise for being heroes whilst suffering not consequences).
With this in mind, we should not therefore be surprised that Labour's support isn't higher than what it is. Contrarily to what some commentators suppose, people do not derive their understanding of current affairs from the collective aether, but rather through reading or watching the news. When that news has a consistent spin on it it's natural that the average person (who does not have the time or energy to delve into it further) absorbs a particular perspective and responds accordingly.
If we want Labour's support to be higher, than more pressure should be put on the centrist press to more accurately and fairly report what Labour's actual policies and positions are. Until then, I don't imagine Labour's support will be going much higher or lower than it actually is.
 This was not, of course, helped by the Labour right throwing a tantrum over Corbyn's support of Freedom of Movement, but that doesn't fit the narrative so goes unmentioned.
 The sheer desperation to pretend that the Conservatives are really, deep down, decent pragmatists whilst Corbyn is a Satanic ideologue has led the farcical scences of some claiming that Theresa May is fighting against Corbyn for a soft Brexit.
 Be under no illusions, if a coup brought Chuka Umunna to power tomorrow I strongly suspect his actual Brexit policy would be no different to the one currenlty being pursued -- if anything it would probably lean towards a harder Brexit.