A link to the page would be helpful.
Understanding where it not working would help. In FSE changes can take places in different places. One thing you can do is use the inspector in your browser to see if your changes show up there. If you see it and it crossed out that means something is over riding your changes. If you don’t see it at all it means your changes where never applied.
If the style is there but crossed out you could try adding !important to the end of your CSS rule. This will cause browser to implement the changes regardless. If however someone has already done this in the CSS code it will select the last !important value it see.
Besides style sheets, WordPress also implements styling in the theme.json file. Also, the block developer could have included a style sheet for a block. If you are using the inspector it will give a link on the rule and tell you where the rule it is coming from.
Also, Keep in mind that there might be a style sheet that is being applied to the backend and not the front end. WordPress allows you to apply different style sheets to the front and back end. As a rule both front and back end apply the styles.css rules but the back end might have a different sheet to style blocks.
Start by using your browsers inspector to get an ideal if the rule is being applied then determine if it being overwritten. If it is try adding !important to your new rule.
