Setting up the backward run can be a little tricky. Your run appeared to produce the same result because it didn't create a new output file and hence the display program used the concentration output file from the previous successful forward calculation.
In addition to the run time, the hours of emission must be negative and the
emission start-stop and sampling start-stop times must be reversed. A sample
configuration is available through the GUI if you retrieve the configuration
"back_conc" from the concentration setup menu.