Hello. You did not present the text to which this question refers, which makes it impossible for it to be answered accurately. However, I will try to help you in the best possible way.
The path between the stores is a place where a large number of people walk. This causes the footprints of all these people to mix or become available one on top of the other, causing difficulties for a specific footprint to be found. Thus, we can say that the policeman was unable to find the footprint he was looking for, because he was analyzing a very busy place, where many footprints mix.