Have you patterned the gun?
What chokes you have in it now?

You say you used a 12 ga and if you could hit them younever had a problem. Then went to a 20 - you can hit alot better but are not kiling them real well. Well the 12 has more pellets to the shot load compared to a 20. So it makes me wonder what choke you are using. A more open choke you will cripple more birds.

You can use 6s but for close shots that might be a problem on quail. I use 6s as in the area I huntas I also have pheasant. But - Quail those littlerockets will sure fool ya. Many timesI was sureI whacked em good only to find they burrowed down real well which made it harder for the dog to find.

JW