Even pros run into issues. Here's a quick checklist to diagnose and fix common problems:
If you are selling or giving away a custom brush set, this tone works best: eyelashes brush photoshop
Natural hair lets some light through and catches reflections. Even pros run into issues
Go to Edit > Define Brush Preset . Name it something like "Natural Flare 01". eyelashes brush photoshop
Real lashes are rarely pure black (#000000). Use the Eyedropper (I) to sample the darkest part of the subject's existing lashes. Often, this is a very dark brown or charcoal.