R MATRIX ALGORITHMICALLY SINGULAR

Greetings! I have run several models and the covariance steps have been unsuccessful due to the following error: R MATRIX ALGORITHMICALLY SINGULAR COVARIANCE MATRIX UNOBTAINABLE R MATRIX IS OUTPUT T MATRIX - EQUAL TO RS*R, WHERE S* IS THE INVERSE OF S - IS OUTPUT Does anyone have an idea as to what this indicates and how to 'fix" it? Thanks in advance for any help/input you can offer! Susan Susan Hudachek, M.S., Ph.D. Animal Cancer Center Veterinary Teaching Hospital Colorado State University 300 West Drake Road Fort Collins, CO 80523-1620 PHONE: (970) 219-7599 FAX: (970) 297-1254 EMAIL: Susan.Hudachek