On both real and fictitious data, the main effects seem to work ok. However, when I add the interaction between subject_ID and rater_ID to test for subject x rater effects, I get output where everything is .000 and a note says that the error component "is set to zero because it is redundant."

I followed up by running a univariate GLM which indicated the SS(error) and df for the interaction were both 0. I am assuming that may be related to the variance components errors, but I don't know how to work around the issue.

I've encountered this problem on a variety of different variables in both real and fictitious data. Any help would be greatly appreciated!