Did you use one of the options with reflections in the 'solar distribution' field in the 'Building' object?
Building, ExampleBuilding, !- Name 0.0, !- North Axis {deg}
City, !- Terrain 0.04, !- Loads Convergence Tolerance Value 0.4, !- Temperature Convergence Tolerance Value {deltaC} FullExteriorWithReflections, !- Solar Distribution
40, !- Maximum Number of Warmup Days 10; !- Minimum Number of Warmup Days
from I/O reference:
Surface Ext Beam Sol From Bm-To-Bm Refl From Obstructions [W/m2]
The solar beam radiation incident on the outside of an exterior surface that arises from beamto- beam (specular) reflection from one or more obstructions. This value will be non-zero only if ?Reflections? option is chosen in the BUILDING object. For windows, the program treats this beam radiation as diffuse radiation in calculating its transmission and absorption.