Unfortunately, a letter from the university is not enough if you don't correct the reasons for rejection. First, you should apply for caips/gcms notes - they contain notes made by visa officers about your file. See these: http://www.cic.gc.ca/english/pdf/kits/forms/IMM5563E.pdf and http://www.cic.gc.ca/english/department/atip/form-imm5563.asp
If you are outside Canada, you have to apply for it under the Access to Informaction Act through a representative. The representative must be a canadian citizen, permanent resident or present in Canada. Read section C and D.