As for Picture Completion, fortunately it didn't factor into the full-scale. In this subtest, a child is shown a picture of a scene and asked "what's missing?". For example, a child may be shown a picture of a volleyball game where a net is missing and he has to identify that "what's missing is the net". Or he may be shown a clock which only has a long hand and no short hand and he has to say that "what's missing is the short hand". Poor environmental awareness, poor attention to detail and a degree of cognitive inflexibility may all cause a lower score. I took an adult version of this IQ test at age 20, and did absolutely atrociously on "Picture Completion", much lower than your DS--it was my lowest subtest. Unfortunately for me, it is a required subtest on WAIS III and factors into all composites.
Both my DH and I are pretty bad in this area as well - I once had a secretary who got really upset with me because she had completely redone her hair (cut, color, etc.) and she said, "so what do you think?" and I couldn't figure out what she had done differently
