The range of the Grabba unit is of 43cm. This range was obtained in free space, with no obstructions or nearby conducting materials. In a test scenario, this involved the UHF tag on the floor (concrete) and holding the reader directly above the tag.
Metallic objects and other sources of interference can reduce the rated range. For instance if the tag was placed on a metal surface it could reduce the range quite significantly. If you have problems, please create as much space as practical between the tag and the metal surface.