Blacks appearing flat in final export how come?

Hello all! 

I have been editing for a while now and have been coming across the issue with my edits, that when exporting from premiere my blacks are flat and not the rich black it is supposed to be. While editing everything appears great. When I take a screenshot via Premiere Pro and save as JPG it appears perfectly the way it's supposed to be. 

But when I export in the usual H.264 codec for some reason the final export makes the blacks appear very flat. I have crossed checked my sequence settings with my export settings, I have checked that the colour space information is all the same (everything is set to Rec 709), yet for some reason that I cannot figure out why, it just turns into a flat looking image. 

On my Macbook it appears perfectly fine and looks great, but the moment I look at it on my PC it looks terrible. I am aware that different monitors mean colours will appear differently and I can live with the fact that my videos won't look amazing on everyone's monitor but I just really don't want my blacks to appear flat because they become noticable when masking or scaling an image down. 

I have checked if it has to do with my VLC player settings, but the video appears the same in Windows Media Player. 

I have also tried exporting with the QT Gamma Comepensation LUT which does not fix this particular issue. 

So to test it out, I just exported my video in Apple Pro Res 4444 and now everything appears perfectly fine on my PC and the blacks don't seem to be flat anymore however, the video is only 40 seconds long and already almost 1GB so when wanting to upload to certain platforms with limitations, this is rather inconvenient. 

I don't know if it is something obvious that I am missing here, but if anyone understands or has an idea what the underlying issue could be that I am maybe just not understanding I'd greatly appreciate the help and advice! Is there still a way I can export in H.264 or any other alternative that gives me a reasonable file size?  

Thank you so much!