PARAMETER ESTIMATE IS NEAR ITS BOUNDARY with NONMEM v6

Dear group, Recently I was running a sparse sampling animal data (one data per animal) using NONMEM v6 & WFN and ran into this trouble: "PARAMETER ESTIMATE IS NEAR ITS BOUNDARY THIS MUST BE ADDRESSED BEFORE THE COVARIANCE STEP CAN BE IMPLEMENTED". I searched the forum for any solution and found the message from Mark Sale (Next Level Solutions). My question: a) Is there any option to overcome this issue without suppressing the message as suggested earlier? b) What causes the above problem? Do I need to avoid specifying any boundary for the thetas? c) What does this message indicate? a poor selection of model? poor initial estimates? narrow boundary? or any correlation issues with the estimates? I highly appreciate your input. Thanks in advance Regards, Chandra Durairaj, Post-doctoral Fellow, Univ of Colorado Denver