I assumed you use the default template. Is this the case ?
I've checked via inspect and the margin commands are active. I've also tried margin-left: auto and margin-right: auto.
Hum... If it's a block and the auto margins are active, then it
is centered. Maybe, its container is not 100% of the width.
Can you test this and take a screenshot like the one I attached. make sure the rules are visible and the image margins are highlighted.
Cras semper augue sit amet dui bibendum
{{:wiki:dokuwiki-128.png }}
Proin tempus felis ac est aliquam condimentum.
{{ :wiki:dokuwiki-128.png}}
Duis eleifend sem vestibulum metus vulputate et sodales erat lacinia.
{{ :wiki:dokuwiki-128.png }}
Morbi leo mi, pellentesque nec pulvinar a, hendrerit ut libero.