Large age, cultural, or educational difference, one person a divorcee, other never married (depending on country), parents not attending wedding, wedding is a simple court marriage rather than traditional, applicant has history of trying to enter Canada, immigrating to Canada, applicant has relatives in Canada, short courtship before marriage, etc.
Basically anything that would seem the marriage is not genuine. If the applicant is from a country like the US, Western Europe, Australia, etc, your spouse is practically guaranteed a PR visa.
