Successful placement of masonry grout is dependent on several critical factors.  These include but are not limited to:

The use of properly graded and proportioned aggregate, maintaining a slump of 8-11 inches, and careful adherence to proper placement and consolidation procedures.

Even if these guidelines are followed, complete filling of grout spaces might not be accomplished if mortar obstructions are present.

For more information, see Keep Grout Spaces Clean, by Jake Ribar, principal masonry evaluation engineer, CTLGroup, Skokie, Illinois.