One possibility is that scp may be testing whether stdin/stdout are connected to a tty device. Your program isn't a tty, so scp could be deciding not to output progress information because it won't be seen anyhow.

If this is the case, you should be able to work around it by using Expect to run scp via a pseudo-tty (ptty).

Again, though, this is hypothetical. scp could be refusing to cooperate for a completely different reason, in which case this won't make any difference, but it's what I would try first if I were in your position.