Why when I sing can I not access the resonance in my sinus and frontal cavities (forehead)?

I'm a baritone, been singing 5 years in musical theater and have a strong powerful voice, but still for some reason I can not resonate in my sinus and frontal cavities. I can resonate in my nasal cavity and anything bellow the bridge of my nose fine but anything higher I can't. When I sing notes at the current top of my range, G or G# above middle C I hit the note but it comes out like dead air or dampered, almost like I was suddenly in a gl box or something. Its like the air has no where to go and gets trapped in my throat its keeping me from extending my range or atleast making those top notes sound any good. Any reasons why? Any excercises to correct this?