The Nonlinear platform is very powerful and versatile. It has many options. Therefore, there are many reasons why you might get such an error message. The objective function, which could be many different functions, is monitored as the parameter estimates are updated. Since the function did not change (decrease), it cannot tell if it converged. So it would help if you could describe the nature of your
model (column formula)
Otherwise it is difficult to provide you a useful answer.