The picture concepts were low on my son's first test because he draws stick figure dioramas rather than detailed, pretty art. The tester decided the drawings were basic, rudimentary, and poorly constructed. Some of that really is subjective.
I just wanted to address this part of a response. There are no written or drawing tests on the WISC at all. The only parts that involve a paper and pencil are the two PSI (processing speed) tests: coding and symbol search.
Ack. You, of course, are correct, Cricket. I was confusing two of the test names. The one I was thinking of was SOI (Structure of the Intellect), Divergent Production of Figural Units (DFU). It is a timed test of the student's ability to draw creatively.
My sincere apologies on my misleading post. I'll be more careful.