The Slow Progress Iteration Limit (SPIL) box on the Nonlinear Solver tab:

is used to terminate the solution process if little or no progress is being made in the objective value.

Specifically, if the objective function's value has not improved significantly in n iterations, where n is the value of SPIL, the nonlinear solver will terminate the solution process. Increasing this tolerance's value will tend to force the solver to run longer and may be useful in models with relatively "flat" objective functions around the optimal solution.

The default value for SPIL is 5 iterations.