have you followed the buddypress advice for making your theme compatible with buddypress? if it's not been specifically designed with bp in mind then things can go a bit wonky.
https://codex.buddypress.org/legacy/theme-development/wordpress-to-buddypress-theme/
if you have followed the bp instructions on making themes play nice then you need to tweak some css. use a browser extension like firebug (firefox) or developer (chrome) to dig down and isolate the id's and classes you need to adjust. play about in the browser as you can non-destructively edit and test things and then when you've found the thing you need to change add it to your theme's style.css file and you should be good to go.