Scapula Stability
If your client's high plank or push up looks like this...
...You will need to focus your sessions on posture and scapula stability.
You may also notice your client's resting posture to look like this....
This "scapula winging" may be due to weak lower traps, serratus anterior, middle traps, or rhomboids (scapula stabilizers).
The proper plank position should be like this...
Train scapula stabilizers, incorporate tactile cues to manually assist your client in contracting the appropriate muscles, and you will improve this technique.