I never used a respirator (stone throws to a minimum, please LOL,) but I AM a pest control guy and can offer a couple tips.
Sounds to me like you're not getting a good seal.
First, make sure the cartridges are placed properly. If they are, facial hair is the most common cause of getting unwanted air in. We used to do a banana test and if you could smell just a hint of it, 90 percent of the time mustaches and/or beards were the culprit. If you are clean shaven, you might just need to tighten the straps. These 3m respirators are really good and you shouldn't be able to smell a thing.