I had this problem for so long and the solution is so very simple. So below is what you see while you work but you zoom in and you see tones. You export and it's still grey right? Well there's only one small step to do.
Go to the layers panel where the screentone layer is. Right click, go to rasterize and click. When it asks to keep original yes or no, select 'Yes' just in case you need to change the dot size. And you should then see this as a result and it should be the same when exporting.
I see this problem often on DA where everything else like materials is showing up screentone and the rest of the art seems to be greyscale painting and it looks odd. The solution is simple. Like photoshop all the layers need to be rasterized for export. I like to put my tones in a folder since it doubles up so much. There is also the method of using tones as a pen that I didn't mention in my first tutorial because it's been covered so much already on DA and youtube. Anyway I forgot about this little tip that proves to be a big help later on when publishing if you publish or even just for posting to the net.
Hopefully this helps and as always don't be shy if you have a question <3 Ask away ^w^