Grounding for the Metaphysics of Morals

In one of Kant’s passages he introduces the categorical imperative and links it with the concept of the good will as an understanding of doing one’s duty in accordance with reason. The moral worth of an action does not lie in the effect expected from it nor in any principle of action that needs to borrow its motive from this expected effect.
In another one of Kant’s passages he argues his point that you should not make choices you couldn’t wish to become a universal law and that you should not make choices that diminish the dignity of others or yourself. This passage proves Kant’s talent for careful analysis.
Kant’s theory still exposes a major flaw every time I analyze his writings. How specific is the situation where we are able to label an action as a universal law? For example if you rob a bank because you have no money, no food and your landlord kicked you out of your apartment because you couldn’t pay the rent, do you denounce that its ok to rob a bank only under those specific circumstances? Or is the situation as simple as just robbing a bank. It’s hard for me to apply the categorical imperative to specific situations because I don’t know how specific the situation is.