Are you using Project Nevada, Onehud or anything like that? If so you might need to use Unified hud project (http://www.nexusmods.com/newvegas/mods/38961/?) to make them compatible. Also make sure your load order is not the problem and that you ticked the darnui esp. Another cause i think is that you have to install all these ui mods in a certain order if you use them all together to make sure that the fonts work correctly.