in my opinion if you had two single entry visas and you followed all the rules on two separate occasions and left the country on time then that should strengthen your case for a multi entry visa. Some common refusal reasons are they do not believe that the person will leave Canada at the end of their stay, you have shown twice that you will.
again that's my view on it, like mentioned all are examined for multi entry and maybe there is some other reason they keep giving you single.