It is unusual for an isolated frontal sinusitis to occur. When it does, it can be a result of previous surgery having caused scarring of the frontal sinus drainage pathway. This can lead to the creation of a 'mucocoele'.
It may even arise due to unusual anatomy within the frontal recess/sinus.
If it doesn't settle down quickly, I suspect a surgical procedure would be required to drain it.
Persisiting isolated opacification within any sinus warrants careful assessment, including surgical exploration if necessary to rule out any growths in the sinus.