BuddyPress.org

@sven. I had this problem in the past, because my mods in bp-custom.php were being overwritten when bp loaded itself.

The details on how to get them to occur after bp has loaded, are in this thread:

https://buddypress.org/forums/topic/remove-menu-from-admin-bar#post-22856

or, as John points out above, they can go in functions.php which is called afterwards.