I wasn't going to reply to this at first, because I have no African Sideneck experience. However, since nobody else has, I'll give you some of my general experience.
First of all...parasites. They very well could be starving due to parasite loads. Any new acquisition should be treated with general dewormers immediately. Many of the meds are available over the counter and are fairly cheap. Even the Vet doesn't charge much for them, but you may have to pay for an office visit in order to have a "Dr.-patient relationship". Either way, it's worth it. Wild caught or captive bred, I treat all of my turtles for parasites yearly.
Turtle appetite varies greatly. I have a Blandings turtle that I have to feed twice as much as any of the others in order to achieve growth. I have no idea why. The turtle is healthy, has been dewormed, isn't more active than the others, isn't bulemic...I just have no idea. Maybe when the turtle reaches adulthood its metabolism will slow...maybe not.
Basically, there is no good formula for how much you should feed. The best thing to do is monitor the turtles' weights via a sensitive postal scale and watch for fat rolls. When the turtle retracts its arms and legs, there shouldn't be any rolls falling out
Monitoring weight is more for making sure that there aren't drastic fluctuations. However, with a little research, you may be able to find out the average adult weight of the particular species that you have. Zoos are good to talk to. They are often very diligent with weighing their turtles, many keep African sidenecks, and most are getting better about sharing information with us.
Yes, I think that it's good to vary the amount of food offered. I only allow my turtles to eat until they're full about once weekly, except for hatchlings and young juveniles.
Two words of warning...speaking from experience. Freeze all feeder fish. Even rosy reds purchased from Petsmart can contain parasites. I learned this the hard way. Full freezing and thawing in hot water should kill most fish parasites. Also, feed pinkies to turtles sparingly. It's tempting to feed them often, since many turtles love them and they're so easy and clean. I've had turtles go off of feeding for two weeks after eating too many pinkies.
I hope this helps. Good luck! I hope that some others with African sideneck experience chime in. Word of mouth gives me the hunch that they are ravenous feeders, but I can't say that for certain.