Just in case none of your programming courses ever explained debugging in the context of systematically designed code, here are basic hints:
If your methods “collude” to achieve a common goal, the “slice” may consist of two disjoint regions of code. You will need to understand the logic that connects those and figure out the fix from there.
In other words, systematic debugging is just like systematic coding. It sounds like it takes extra time but in the end it will save you time because you’re guaranteed to get to the goal.