Hello all I used to lecture in art and know a fair bit about it.
All of the artists pictures shown here have a few problems in common;
The colours seem very reduced. This is partly because some of the photos are taken in artificial light, this really cuts down the range (or gamut or colour space), then on top of that the camera doesn't grab all of the colours, and then as if that weren't enough some more of the colours are lost on screen or through the painting process. I would strive to get natural lighting first of all in the photos, because even experts are really really going to struggle to do anything with these. Ideally if you can get soft sunlight - sun through partial cloud this will really bring up the mix of colours.
Okay soo.... so when you have more coloured areas, it's easier to paint, you have a more interesting surface, and you can 'sculpt' more because some of the colours will describe contours and areas of the face. You won't get large areas of very similar colours which don't really work in face paintings.
The next thing 'for the next level' is the use of grounds, this is underpainting of a different colour. Actually using a cool blue is good for portraits - it helps stop a face getting too warm, overloaded with hot colours.
And the paintings seem very smooth, generally a bit of graininess is used - eg canvas. It allows for a lot of interesting effects and gives a surface vibrancy to the picture.
As you paint sense the structure underlying the face. It sounded very weird to me when I first heard this, but the knowledge you have of bony areas and so forth really helps assemble the reality.
One final trick for now is to actually think of yourself as a sculptor with the brush, try and sculpt form - the more you think in 3d the more the painting will gain depth.
Okay if you want to read more I would recommend 'The Art Students Handbook' by P Newberry which contains many more pages of this kind of stuff! All the best to you.