The Kantian ethical formulation, "Always act according to that maxim whose universality as a law you can at the same time will," seems rather vague. Where, exactly, do we decide how to formulate one's actions?
Suppose a poor man steals medicine he otherwise can't afford, in order to save his child. Which maxim's universality as a law, exactly, should he be willing? If we examine the maxim as being "It is permissible to steal", then we clearly have a universal law that shouldn't be willed, but if we examine the maxim as being "It is permissible to steal in order to save lives, when there are no other options", then we have a maxim that might hold up under more scrutiny.
For any given action, there are multiple possible maxims that are more or less reprehensible, each of which can justify the action in question. So how does Kantian ethics treat situations like this? Do we boil it down to the most semantically/logically simple maxims? Do we go instead for the most nuanced maxims?