我在运用stata软件处理数据时，出现以下结果：
svy:tobit t x,ul(1.0e-21)
(running tobit on estimation sample)
convergence not achieved
an error occurred when svy executed tobit
错误提示信息：r(430)
You have estimated a maximum-likelihood model and Stata's
        maximization procedure failed to converge to a solution;
        see [R] maximize.  Check if the model is identified.
我想请问一下：在什么情况下会出现这种错误？怎样可以解决？