Shyam,
What is not working ?
Forecast Horizon is a kind of fictitious concept that uses a convenient "logic" for calculation of TOTAL DEMAND.
According to SAP,
Within the forecast horizon, Total Demand = sales orders quantity (or customer demand quantity). In other words within the forecast horizon, forecast is not considered as demand. Even more simply put there is no fun is planning against forecast when you are in a time horizon where you are supposed to deliver whatever demand you managed to generate against the forecast. This is also partly configurable using the forecast consumption settings. In total demand there can be other demands like dependent demand and distribution demand,
Beyond the forecast horizon,
Total demand = Forecast + sales orders !
So, you need to evaluate the results in conjunction with SNP production horizon and other settings to see if the arithmetic is as expected by you i.e. what is working and what is not working.
Note all this is configurable. Important thing is SNP can only plan for products with make to stock planning strategies because it is only for make to stock products that SNP makes business sense. This is the reason why SAP gave you the feature of being able to control what your total demand SHOULD be within and beyond forecast horizon. Not all may agree with this. So people change this logic. See if in your system someone did something non-standard.
Hope this generic answer helps your research
Thanks
BS