If the calculation is not complete, the Calculation status box indicates this, and if you click on the Chk button you can open the error message in a text editor in order to analyse the error.
For some types of error, row numbers in the netsim file where the calculation module has encountered problems are written to the Chk file.
Open the calculation file and review the rows excluded.
There is a list of error messages at the end of the manual. You can see here what an error message means.