Note that this ko is fairly heavy for White, although conventional wisdom tells us that the ko is heavier for Black. The reason is that if White loses the ko, the loss is quite substantial.
The descent at fails. The cut at
is tesuji, and Black gains sente to live at
. Now White cannot approach at a and capture the three Black stones.
The connection at fails. After
makes the second eye, White is unable to play at a, so Black wins the capturing race at the top.