With the new SR system, being it randomized, the store robberies start with a percentage of 5%. It means, if two or even one cop are near the store robbery at the time it appears, or got a property near, the store robbery can be stopped without criminals having a chance to fight for it.
Now, my suggestion is to delay a little bit the notification cops get about an ongoing store robbery so criminals get a chance to reach the store robbery. It can be done by two minutes delay or by percentage delay (or even both, maybe). Keep in mind, reaching 0% from 5% is technically an easy task, compared to reach 100% from 5%.
Quick edit: Cops also should be notified when the SR reachs x amount of criminals on it, f.e 5.