As a working chemist who does chemical reactions everyday, I can tell you from experience that there are several reasons why a reaction's percent yield can be lowered. First of all, the percent yield of a reaction is the actual amount of product obtained divided by the theoretical amount of product possible (if every mole or reactant was converted to product) multiplied by 100. The highest possible yield is 100% but this is rarely found in practice. First of all simple accidents such as spills will lower the yield. Often times simple mechanical problems such as not being able to remove every particle of solid from a piece of glassware or not being able to extract all of the product from a solvent using liquid/liquid extraction will lower the yield. But most often percent yields of reactions are lowered due to the formation of byproducts, or unwanted alternate products other than the desired product. Every mole of byproduct formed will take away from the yield of the desired product. Even relatively simple chemical reactions can often produce byproducts in small quantities.