I won't repeat my comments from the other thread, as it sounds like you have read them already, but those are all pretty much in play for your DC's situation.
Factors that affect focus are definitely places to look, including the ones already mentioned (medication, time of day, examiner/rapport). Another factor that particularly affects the VCI in dyslexic children is cumulative reduced exposure to deep reading vocabulary, which affects not only the vocabulary subtest, but also the similarities subtest (can't reason verbally if you don't know the words). Not uncommonly, I see small to moderate declines in VCI (mostly in the vocabulary subtest) over historical numbers in my adolescent students with a history of reading difficulties. With the removal of comprehension from the VCI, knowledge of academic vocabulary also becomes even more important.
BTW, 37th %ile is not technically super low (except in comparison to Very Superior scores), as it's not significantly different from the 50th %ile scores (scaled score 9, as opposed to 10). All solidly in the Average range. I'd be interested to know what else was at the 50th %ile. Were they the other CPI subtests? (symbol search, digit span, picture span)
