In EnergyPlus, this warning means one or more of the air loops did not converge. This usually happens when a HX is used in the outdoor air system, which can vary the mixed air temperature, which changes the air loop flow rate required to meet the load, and the simulation hunts for an answer. If the air loop coils can tightly maintain a fixed set point temperature, these warnings are less common. If these warnings are not prevalent, you can probable disregard them. If there are many, you should probably investigate the cause. You can report out some diagnostic information using Output:Diagnostics,DisplayExtraWarnings; However, even the most experienced users find this data hard to interpret.