Okay no expert here, but a quick suggestion.
Drybrush in more layers. Start with a colour close to your basecoat and drybrush as normal.
Mix a lighter version of the colour then drybrush again, but this time wipe more paint off your brush then before.
Mix an even lighter version, and drybrush very lightly, with even less paint on your brush.
Mix a very light colour, perhaps even white and very lightly drybrush just the edges, and wipe all the paint you can off your brush first.
Or paint on the highlights instead of drybrushing.