This can always happen to such a complex numerical like ENVI-met. We have invested a lot of time to increase the internal intelligence of ENVI-met. The recent version can handle a lot of trouble situations by itself and find a solution to ensure a proper model run. However, sometimes things simply don't work, and here are some clues what to do:
Don't be sad or frustrated! Remember that you are working with a complex numerical model. 10 years ago this had required a million euros super-computer. Only because the computer you use was much cheaper, that does not mean that the program has become easier to handle. It's the other way round.
Try to extend your model area via “Nesting Cells” if your have trouble with the flow.
Move complex buildings away from the model border.
Extend the vertical extension of the model.
Decrease the time step if the model gets unstable in the normal calculation loop. (See
[TIMESTEPS]-Section of the manual)
Consider simplifying your model. It's not the real world either, so you can adjust some very complex corners to an easier configuration
Also have a look at the warnings and information in the ENVI-met program output.
If your model is not working, send the input files (.in and .cf) to me! I love to receive mails and also it helps to find bugs and improve the program!