Do you run a transient model? If yes, does the time-step size increase or is the time-step size relatively small (e.g. < 0.1 d) ? You may check the time-step behavior in the Time-Step History. In the main menu, please click on [b]View[/b] and then on [b]Charts[/b]. After that, please open the [b]Time-Step History[/b].