You will be able to see rejection reasons in detail, for example (this is the case where my cousin mother's visa was rejected).
"Application and supporting documents reviewed. The applicant is seeking entry into Canada to visit her daughter. The following information was noted during my review of the documentation presented: - The applicant has limited or minimal funds according to documentation provided; -Host mentions he will cover the costs but no supporting document is provided; - The applicant has a declared no previous travel history. As a result of my careful review and considering all relevant factors presented, on balance, I am not satisfied the applicant is a genuine visitor who would comply with the conditions of temporary entry. Application is refused."
So, you can clearly see what Visa Officer's concerns are, you addresses those concerns and hopefully it will be approved.