How come nobody ever recognizes the Senshi in costume?
There’s no real explanation. They may be magically immune to being recognized, but we’re never told that. In Japanese episode 5, and in dub episode 15, Usagi hides from her brother when in costume, apparently because she’s afraid she’d be recognized. If there is magic involved, she didn’t know it at the time. Dub episode 31 says that they can’t be recognized in their normal identities, though this line may not have been in the original version. Asai, in episode 100, recognizes Venus. Fiole in the R movie recognizes Tuxedo Mask. This problem doesn’t happen in the manga; they seldomly appear in front of people they know. Usagi and Tuxedo Mask recognize each other almost immediately, and Motoki recognizes them in costume (volume 3 p.p. 100, 177; volume 4 p. 113).
