The red flag from your application is "diploma supply chian managemen "
This is not inline with the natural progression of your career as a student and engineer with 4yrs of exprience, the immigration officer has grounds to believe that you're taking this diploma course with the intention of getting your graduate work permit and immigrating to Canada. I think this is why your application was rejected.
you should instead apply for graduate school, something that's related to your major would help you strengthen your case.
what did you write in your explaination letter ?? did you write why you're changing career direction after 4 years as an engineer ?