Many students lose focus on each step. They question the reasoning behind each of these steps as they should. Unfortunately, this questioning can last more than a month. It takes this long for an algorithm to be firmly established. They have difficulty synthesizing many small steps into one large one.
Stoichiometry involves a grand synthesis with many higher order thininkg skills.
Third, part of the chain involves mole ratios which is a magnet for math mistakes. As you know, a mole ratio is determined using coefficients from the two specified components from the balanced equation. Most students often encounter confusion with these ratios. They pick wrong values or have the ratio inverted. Fourth, traditional stoichiometry quickly tries to get students doing all of the different variations on a theme using matter presented in different quantities.