As mentioned, the BB is of irregular diameter – as you put them in the magazine, don’t dump them in let them flow inspecting the BB’s as they pass from the container to your magazine. By doing this you can usually spot irregular and odd-shaped BB’s and remove them before you fire them thru your hop up. I’ve had VA BB’s that look like bullets, which if not caught when putting it in the magazine would have jammed the magazine or jammed the rifle.
Or maybe you're using too much hop up. Turn off your hop up, fire a couple shots – see where your BB’s are going and adjust accordingly. My JG SiG 552’s hop up is very forgiving, if there's not enough hop up, the BB will just roll out of the barrel. I adjust the hop up according to that; once the BB stopped rolling out – I fired until it was straight. I was having jamming problems with this rifle until I realized that it was due to too much hop up used.
Due to availability in my area, a lot of airsofters use Walmart BB’s, and once you understand the dynamics of these two important steps – your jamming problems will be a thing of the past. IMO - there's nothing wrong with Walmart's .20 Bio-BB's... run what you got!