Are there limitations to how often I can concuss my opponent?

Solution 1:

Mr. N's neck pillow protects him against one headshot per fight.

Mr. N's neck pillow protects him from one head shot every fight, preventing him from going into the concussed state that would normally follow a headshot. However, any subsequent head shots in the fight will concuss him as normal.
- Divekick Wiki - Mr. N

Mr. N is the only character who can cancel a headshot. Once his neck pillow is removed, there are no limits to how many times you can headshot him.

Mr N. with his neck pillow