What stores are you robbing/how are you robbing them?
The 24/7 convenience stores. The first time you walk in it tells you how to hold em up but just point a weapon at the guy behind the register...
it's random what happens next. He either pisses himself and gives you a bunch of cash in a bag, runs away screaming or pulls a gun and tries to stop you. In either of the later two cases (after killing the clerk) get behind the register (like you're the clerk) and hit right on the d-pad. You'll grab a few fistfuls of cash.
it seems you get more money if the cashier does it for you and if you have to grab the cash yourself the cops start coming immediately as well.
very early in the game i had been busted and lost all ammo and had almost no cash so I was desperate looking for some money when I did my first robbery. no ammo in my pistol and the dude still coughed up the cash.
one guy started begging me to leave saying I had all the money already when i noticed the back room. i walked in (cops were coming) and saw
a medium sized safe. i can't wait to get c4 or figure out how to open those up!