cover image not responsive?

Hello,
I’ve just discovered that the image I planned to use with the cover template is not really responsive : I only see what’s in the middle of the image (and there’s nothing in the middle of my image).
Is there something I did wrong.
I would prefer to see the whole image, horizontally, and not a third or fourth of the image covering the phone.
I’ve tried the same page with the twentytwenty template, and there was no problem with the cover image, it was totally responsible.
maybe i should switch template?
many thanks in advance for your answer, and thank you for your incredible work.
v