Stocking big fish is always risky, but to stock them and call it "job done" is madness.
A fish of upper 20 or 30lb has had things its way for 8 years or more on a fish farm. Lots of space, food and usually a clean clay pond, so they're used to having their pellet ration presented cleanly infront of them daily.
For example a 30lb fish fed 3% bodyweight/day would be fed around half a kilo/day everyday. Thats 3.5kg a week to feed 1 fish.
So when stocked into a silty lake with the added stress of competition and anglers, and the daily feed ration no longer presented on a clean clay substrate, is there any wonder these fish struggle to hold there weight, and often turn belly up.
You are expecting a huge fish to find 3.5kg/week by digging through the silt to find "naturals" to sustain its bodyweight. And these beasts are capable of eating in excess of 5%/day!!
Thats why smaller fish tend to do better, because in a fish farm you can stock a large number of fish and still keep a healthy biomass in each pond. More fish, therefore more competition for food when fed, so when they're stocked out they are used to searching for grub aggressively.
If your going to stock big, give it to them their way. If you put an obese man out in the jungle, who's had it his way on benefits, would you expect him to thrive?