How to solve it? ERROR: PARAMETER ESTIMATE IS NEAR ITS BOUNDARY

Dear NM Users, Currently, I am running some Pop PK models in NM VI. Sometimes, I hit the following error message: PARAMETER ESTIMATE IS NEAR ITS BOUNDARY THIS MUST BE ADDRESSED BEFORE THE COVARIANCE STEP CAN BE IMPLEMENTED. This error message is self-explained, but when I checked the final parameter estimates, none of which is close to its boundary. Sometimes, if I remove the boundary, it will work, but not always. Could someone here give some suggestions to help me out? and point out what is the possible reason to cause this error message? I appreciate your input. Thanks a lot, Jian --------------------------------- Park yourself in front of a world of choices in alternative vehicles. Visit the Yahoo! Auto Green Center.