Why is your percent yield of Aspirin less than 100%?

Unless you have 100% pure acetyl salicylic acid (same as aspirin), you will not get 100% yield. This is because commercial aspirin samples have fillers (called excipients in pharmaceutical language) such as starch, sugars or other material in which the drug molecule is dispersed. This is to provide bulk to the material. Based on your results, it is quite apparent that the sample contains about 43% other material and only about 57% aspirin.