So many things to choose from, I'll just pick two:
Firstly, he doesn't have a direct mandate because he hasn't faced a general election as party leader. The British constitution allows this, but the public doesn't like it and doesn't tolerate it for long.
Secondly, a lot of the things that need doing in Britain involve undoing things that he did when he was Chancellor (Finance Minister). Everyone knows that, but he can't admit it, so nothing gets done.