Dropdown menu styling problems (cut off)

Is there anything that can be done about dropdown menus being horizontally cut off in several themes, including Blogs.MU and Social? This happens especially with certain plugins (eg, Buddypress Activity Privacy). It seems as if there must be some issue with space... some limitation, even though there's plenty of room there.
This does *not* happen with Buddypress Default or any BP Default child themes I've tested.
Can someone point me in the right direction - some setting I might play with?
Thanks in advance!