I would really suggest to follow the advice below and similar strategies.
I completely agree. A lot of people focus on their CV (obviously this is a crucial part of your application) but perhaps spend less time on the small details of their application such as wording, grammar and structure. Most applicants are quite competitive if they've made it to national competition so the difference between your application and a colleague could be down to an eloquently written proposal.
My old institution ran a tri-council writing seminar where they provided a lot of great writing tips for the proposal and contribution sections. Most of these tips are mentioned above. It would be worth while to inquire with your institution to see if they run similar sessions or if the office that collects NSERC applications can provide any tips/feedback etc. A mentor of mine suggested paying close attention to the optional documents you can provide: inclusion in research, explanation of delays, etc. Even if it may be obvious why your program was delayed because of COVID (as an example) its a bonus to include a blurb acknowledging these aspects and how you persevered despite this.
As mentioned above, asking your references to highlight certain aspects of your research/volunteer/academic experience, your proposal or general attributes may go a long way to complimenting your application package. While they may be writing you a positive letter, you may take it for granted that your referees are being specific, which can emphasis some of these points (particularly if you've run out of space in the relevant sections).
Unfortunately, application committees are made up of people who have biases (whether intentional or not) and when faced with a stack of excellent applications choices have to be made. Rejection from NSERC does not determine your worth or your value as a scientist - speaking from someone who was rejected on my first try. Good luck to everyone hoping to apply again!