The most likely issue is that the X-T10’s eye-detection AF is limited, especially when the face is smaller in the frame, less well lit, turned in profile, or moving.

To improve results:

• Don’t rely on eye detect when it’s missing the target. Use normal AF and place the focus point on the eye/face yourself, or use manual focus.
• If the subject is still, manual focus in magnified live view is the most reliable method.
• Make the face larger in the frame when possible; eye/face detection works better when the camera has more facial detail to lock onto.
• Improve light on the face and ask the subject to stay still when possible.
• Stop down a bit more if needed. A smaller aperture (higher f-number) gives more depth of field, which helps if focus is slightly off.
• Shoot multiple frames; with movement and blinking, a short burst increases your chance of getting one perfectly sharp shot.

In short: for critical portraits on the X-T10, regular AF or manual focus is often more dependable than eye-detection AF.