Does your crew not use radios? This is a very good reason why we all should. Did anyone else on the crew flag it? B should have seen the subs as well, and S (if 7-man crew.) Especially if either one of those flagged it.
If this were me, and it's obvious that H and/or R are not going into sub mechanics, I'm running towards U to get him to hold, and if that still doesn't work, I'm blowing it dead to allow the substitution to occur. If your crew has a problem with you doing that, you need a new crew, because you're the one being a crew saver here, to fix their mistake in not seeing the sub. BY doing something, you're at least making it defensible on film that at least one person on the entire crew saw it and tried to correct the error.