Firstly, my apologies for not responding to your contributions earlier. I was expecting to be email-notified of responses in the thread the way I had been the previous time I asked something here, but this time that didn't happen. Thanks for explaining the deal with the G-matrix and the zero covariance. It is indeed the case in my analysis that all the individual random-effect estimates/BLUPs are reported as zero for the category pairing with the zero variance component, i.e. C vs A. Such a result continues to seem implausible to me, but it is reassuring to hear that an error on my part is not the cause. As for Steve's speculation about the possibility of variance being "stolen" from the other category pairings and misattributed to B vs A, I can confirm that B vs A actually exhibits more variation than the other pairings. In other words, its larger variance component makes sense from a subject-matter perspective.
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