Five ounces is not the "max," it is the "average" for normal gravity beers, if you are priming them when the beer is around 70 degrees. It produces roughly 2-2.5 "volumes of co2." If people carb to style and use brewing software, they are often priming with larger amounts of sugar. For example many Belgian beers are primed higher than 2.5 volumes oc co2, and therefore require more sugar; but also usually are bottled in thicker bottles.
Some styles are carbed as much as 4 volumes of co2.
I can't remember where, but when I was bottling my saison this year I was concerned about the max co2 capacity of 12 ounce beer bottles, since I planned to bottle six standard 12 ounce bottles for contest entries. And I was well within the saftey range of the bottles. IIRC standard beer bottles can handle 6 volumes of co2, but beyond the range of most levels of carbonation for most styles.
But my suspicion, since you are a new brewer AND you are priming with what is the correct amount sugar for the majority of beers, is that if the carbonation is not to your liking, more than likely you are not waiting the
minimum 3 weeks @ 70 degrees, that the average beer needs to carb and condition. Usually when someone says their beer isn't carbed enough, it is becasue they're openning their beer too soon.
I wrote extensively about carbonation here;
Revvy's Blog, Of Patience and Bottle Conditioning.
Before you add more sugar, I suggest you take a read of that blog and follow the suggestions in there.