In short, critics argue Starmer is falling short because:
He lacks a strong defining purpose or message, making it hard for voters to connect or rally behind him.
His public popularity is weak, undermining legitimacy and momentum.
He is facing internal cracks within his party, meaning leadership may be perceived as weak.
On policy he is under pressure: both delivering on big promises and avoiding backlash.
His balancing act between left and centre (and business interests) may leave everyone somewhat dissatisfied: the core base feels let down, new voters are unconvinced.