Block design is timed, so it is possible to complete all items correctly but not reach the maximum score.
However, if your child has two or more subtests in the 18-19 range you could ask about the extended norms. No further tests are required, the psychologist will simply use the raw scores to look up the extended norm scores. Note that with extended norms it is possible for a score to actually go down, not up, as they are a completely different set of norms. My DD had extended norms applied to 4 verbal subtests (her psychologist had administered all 4, to see what she scored on the other two, after she hit the ceiling on the first two), three went up, one went down. The VIQ then went up overall as well.
