Why are yields of some chemical reactions so dependent on the stoichiometry of the reagents?
The proper stoichiometry may not be the only reason for the friend's success, but also the overall preciseness of their preparation procedure.

Chemical reactions are not generally that much critical, but there are lot of  cases, that are. 

There are few aspects to consider:


Excessive reagents may affect the effectiveness of the product isolation.
Excessive reagents may alter chemical environment in final mixture, affecting formation of products.
Excessive reagent may not remain, but may form another product, instead of the desired one 


Typical case for 3. is e.g reaction of $\ce{NaOH}$ with $\ce{H3PO4}$. Phosphoric acid forms 3 salts. 

$$\ce{NaH2PO4 + Na2HPO4 + Na3PO4}$$

Let suppose you want to produce 
$\ce{NaH2PO4 }$

$$\ce{NaOH + H3PO4 -&gt; NaH2PO4 + H2O}$$

Extra  $\ce{NaOH}$ will create from the wanted salt  other salt...

$$\ce{NaOH + NaH2PO4 -&gt; Na2HPO4 + H2O}$$

... and the amount of the final product decreases.

More detailed answer depends on providing your particular scenario.