Did the work experience letter that u attached b4 had job duties and responsibilities mentioned? If not, then u need to add them. Did it mentioned work hours, salary, job title, duration of the job?
Were the exp letters on official letterhead? Anything u had doubt about but u thought that it will not cause an issue shud not be ignored now.
If you cant get more detailed exp letters, show them proof that you tried talking to hr/manager abt detailed exp letters, but they denied giving it. Show email conversations as proof, show ontario anything that is written or discussed using official channels.
If you can get anything frm ur supervisors, get it on a blank paper in detailed manner. Attach their official business card with the letter.
Dont worry, u hv enough time to convince hr or supervisor. Dont lose this chance, u hv got everything. Best of luck.