Colors issues between Mac and PC

Hi all,

I'm having a bit of an issue at the moment , I have a photo studio at home and most of the time I have to darken some part of the black background , and all it looks fine on my calibrated Mac and on my I calibrated MacBook and all my other devices at home.

I don't own a PC and so far only one client has told me this.

Some of the edited photos specially the black backgrounds she is able to see the brush strokes.

I'm wondering if its something to do with the PC's or maybe it will show to all my clients.

Thanks and any help is appreciated.

I will post a couple of the pictures soon.