I've sorted out a list of professors from few Canadian Universities and I've sent customized emails after reading their papers and detailing in short on why I am particularly interested in their research and how it relates with my goals. This took a lot of time for just one email. Most have replied by saying they don't have suitable projects in their labs or are not accepting Masters students. I am bit worried now that I am nearly towards the end of my list of potential supervisors. Should I find more professors and broaden my research area of interest, as suggested by some senior friends? I am an international student and don't have any publications although I do have research experience. Could my lack of publication be a factor for all the rejections? My average GPA is 3.6/4.3 and my field is Biology.
No, do not keep emailing the professors.
Often professors get too many emails, and they may not have the time to reply. It might have been helpful to see a sample email. What information were you asking them? What were you expecting out of emailing them?
Professors are quite limited with funds and resources (or already have a full team). The likelihood that someone who emails out of the blue to be a suitable choice is slim. Some places probably need a commitee to hire, as a safeguard against discrimination and errors.
You are far better off looking for universities with open positions, I think, or strengthen the connections with professors who already know you (and might write a recommendation letter for you).
Also, publications are way better than grades, research experience is very valuable (at least in maths, I suspect other fields are similar).