I don't think the left are starmers biggest problem, they either reluctantly voted Labour or didn't vote.
The problem starmer has is those voters who don't really follow politics who in the past voted Labour without much thought as it was what many of the working classes do, the tories represent the rich etc, then along came brexit and since then they see labour as the party that tried to block brexit, the tories keep stoking that fire and they foolishly believe they have more in common with the old etonian establishment, the icing on the cake for tory HQ was a bumbling fool for PM who can hide his privilege behind his stupidity.
Will take long time for Labour to win them back.