Why my buck converter is not performing well?
My first thought is the distance C21 and C22 are from U14. The ground loop for C21 and C22 is also unnecessarily large too. Add a capacitor as close as possible to U14 between pins 1 and 3.

The reasoning behind this is that for a buck converter, the full load current is seen as ripple at the input to the converter. Any stray inductance between the converter IC and the input smoothing will cause voltage spikes at the converter input. These voltage spikes will increase as the output current increases, and are worse for higher switching frequencies. The Absolute Maximum voltage input for U14 is only 19V.

You could also try lowering the input voltage to 7V or so and trying that as an experiment, if you can't get a capacitor closer to U14.

Good luck!