Labour got 9,708,716 votes in 2024 vs 10,269,051 in 2019. Starmer and Labour did not convince voters adn lost votes to the Greens.
What happened is that people did not vote for the Conservatives and instead voted Lib Dems and, especially, Reform UK, which got a massive 14% (3rd place and more than the Lib Dems). The Reform UK vote is because the Conservatives did not deliver on Brexit and even more importantly did the opposite of what they said on immigration, which reached record level.
The number of seats to Labour is a result of the above (Conservatives dropped so Labour candidate was elected) not because people voted Labour more than before. The surge is Reform UK.
So the same issues that have been at play in the Brexit referendum are still the key issues.