What problems arose among the pharisees in regulating the sabbath observance?

The Pharisees' legalistic approach led them to creating an ever-more complex system of rules, and their extra-dutiful observance of law focused on external obedience to rules more than internal attitudes of the heart.

Jewish rabbinical tradition counted 613 commands stipulated in the Law given to Israel by Moses.  For the Pharisees (meaning "those who are set apart"), that wasn't enough.  They sought to set themselves apart from the common man by the way they applied the Law to every detail of their lives, making their own specific rules for specific situations.  So as new situations arose, new religious rules were imposed.  The Pharisees' body of law was something like the US tax code in that way!  They gave particular focus to all the ways that one should obey the rule of resting on the Sabbath.

In the process, the Pharisees also paid primary attention to outward adherence to rules.  The spirit of the Law as originally given was aimed at conforming persons' hearts to the ways of God.  But following the laws of the Pharisees became more focused on maintaining outward consistency with the rituals and regulations they had established.  In regard to the Sabbath, the original intent was so that people would stop other activities in order to give full attention to God and his Word.  For the Pharisees, the focus of the Sabbath became more about regulating how much activity was considered allowable or not in different situations.