Covariance structure question for mixed model repeated measures ANCOVA

I'm working on a mixed model repeated measures ANCOVA and am working on identifying the covariance structure for my model. I am interested in modeling 2 of my variables as repeated measures, and am wondering if this is even possible. I'm working in SAS, and keep getting error messages telling me that each of the 11 covariance structures I've tried are unable to handle multiple repeated measures variables. Is this a limitation of mixed model repeated measures ANCOVAs, a limitation of SAS, or a limitation of the covariance structures that I've tried so far?