Faulty components function within SMPS leading to be weak current

Is this a homework or course question?

Assuming it's a genuine question?, then high-ESR electrolytics would be the most likely cause.

For a buck converter only component that connects the input to the output is the inductor.

Another guess, and it is only a guess: Current limiting is triggering early.
Depending whether this function is implemented on the primary or the secondary, how the current is sensed (shunt, transformer, hall), the controller’s CS implementation, peak or average limited, and other details, it could be a couple dozen different things.