I've already gave you some suggestions on the other topic, but I want to give you an explanation.
First, I hope you have been suspected for sinusitis, where frontal (forehead) sinus is inflamed or infected.
When there are additional symptoms like nasal (nose) discharge or congestion (stuffy nose), diagnosis of sinusitis is obvious.
If the sinus is completely obstructed, the headache could be the only symptom.
As I said in my previous post, this headache could be neuralgia (pain that arises from an irritated or damaged nerve).
Since branches of the trigeminal nerve are responsible for conveying pain sensation from the eye and sinuses, disease of these structures (sinus, eyes), may cause a headache in the center of the forehead.
You may have frontal sinus polyps, too.
The trigeminal nerve can become infected by the virus of shingles (herpes zoster virus). Compression or irritation of the nerve fibres, commonly caused by the progressive lengthening of a branch of an artery as it hardens with age, multiple sclerosis, and malocclusion, can be the cause.
Best wishes!
Marija