Masking Issue – Showing transparent on Youtube

Hello everyone.

New on the forum, mainly because i wanted to ask some advice on the issue im currently having.
I have been given some footage of a pianist, i have been asked to fix the issues it has, as the ‘finished’ product he was given wasn’t at all finished.

A big problem was a white power lead trailing up the side of black piano… didn’t look good. I did a very simple mask to overlay to hide the cable.

All looked great on my iMac… exported for youtube, looked fine on youtube, until the guy looked at them on his laptop and the
are where i covered the cable is showing as ever so slightly transparent. You can see the cable under the mask.

I checked to obvious, had i caught the opacity or something, but no all fine. The link is here –

As i say, it looks fine on my imac, phone, ipad… but on his and my friends pc, it shows up.

Can people see the white cable on the right hand side? If so what the hell could it be? Why showing on some screens but not others?

via Video Editing Forums: Digital Director – Adobe Premiere, Premiere Elements, and After Effects