Wrong poll.
Voting intention polls have a load of holes in them, not least the trouble with modelling our FPTP system (see the 2016 US election for a similar issue with constituency based voting systems and national polls).
Consistently the most reliable indicator is the “Who would make the best PM” poll, and yes he’s being beaten to a pulp in that. Not just by May but by don’t know as well.
I mean the fact that he’s in opposition against the worse government in living memory and still can’t get a lead also shows we won’t win. Opposition peaks outside of elections. To win you have to be well well ahead to account for the incumbent bounce the government always gets.
Again, I’m saying this despite the fact I want Labour to win the next GE and I voted for a Corbyn in 2015.