Try increasing glass optimization level under Global Illumination> Details:....
And then increase the Reflection depth under Details>General Options:-
(also a good idea to increase Ray Depth while you're at it; anything between 15-20 should give you decent looking glass reflections)
Make sure you use a glass material with good reflection values and settings otherwise this will not work.
Ideally they work with the default glass you get in the Materials Catalog available with AC18