Can't see server hit shuttle?
Rule 9.5 of the Laws of Badminton (Part II Section 1A) states .....
In doubles, during the delivery of service, the partners may take up any positions within their respective courts. which do not unsight the opposing server or receiver.
So what does this mean?
The most common time it occurs in mixed doubles where the lady stands in front of the gent when he is serving -- the receiver must be able to see the shuttle make contact with the server's racquet -- if he can't he should not return the serve or make any attempt to hit the shuttle - and ask the player obstucting his vision to move.
The umpire is in no position to notice the obstruction and cannot request such action.... if however after the receiver has asked for his opponent to move -- that player does not move -- the receiver is entiltled to ask the umpire to request the player to move....... should the player then not move the umpire is able to issue an official warning (displaying a yellow card).
A couple of other points to note -- the umpire is the only person (in the absence of a service judge) that can call a fault on service. And the umpire is also the only person who can call a let.....it is not automatic for a let to be called when another shuttle comes onto the court you are playing on.