mirroring works through my phone but only the sound works from my mac. It has worked before but now suddenly it doesn't

I can only get the sound going when mirroring through my macbook. it worked before but now suddenly only sound comes through. There's no problem mirroring my iphone or my ipad. then i get sound and picture. it must be a setting. what have i done and how do i sort it out?

when mirroring the screen just goes black.