We are from a small manufacturing unit and do roasting and packing of cashews. After roasting we pack them in polypropylene pouches with nitrogen flushing. But after 2-3 months cashews are smelling bad when packets are opened. Kindly help us learn how to overcome this issue.
Presumably you are experiencing rancidity, due to oxygen.
While you have nitrogen flushed the bags at filling time, the bags are not impervious to oxygen diffusion.
A different type of bag (an "oxygen barrier" bag material) or package (glass jar, or metal can) and, in addition, (based on other packaging I have seen) oxygen absorbing packets inside the bag may be required for long shelf life. Either alone would probably extend the shelf-life somewhat.
Temperature of the storage area can also be be a significant factor in terms of storage longevity. Nuts will last much longer kept quite cold ( 4 degrees C or less) and even longer when frozen, than they do at room temperatures. Some storage facilities can get very warm at times.