Do you have cubemaps built? That is the general issue with glass and other reflective surfaces.
You should be placing one at roughly eye level in each separate room/area of your map. Read up on cubemaps on the VDC to understand what they do.
The amount of cubemaps depends on the amount of different reflections you need. So if it's a surf map, one or two will probably be enough
Just Place env_cubemaps with [insert distance here] away from each other.
Also this is a good read : http://developer.valvesoftware.com/wiki/Cubemaps