SAS proc nlmixed，出现警告：WARNING: The final Hessian matrix is not positive definite, and therefore the estimated covariance matrix is not full rank and may be unreliable.  The variance of some parameter estimates is zero or some parameters are linearly related to other parameters.看网上说是Hessian 矩阵非正定问题，那这种情况我接下来该怎么解决？