They build what they think they need.
If they haven't built more, it's because they are busy rasing brood and gathering pollen. If they get caught up on those duties they will be putting away the syrup (instead of just feeding it to the brood) and they will need more cells to put it in and they will build more comb. Right now a lot of them are tied up just making heat for the brood and caring for the brood. They were quick at first because they need comb just to raise any brood.
I wouldn't worry about it. When they need it, they will build it.
You are already doing the only really usful thing, which is to feed them.
If they haven't built more, it's because they are busy rasing brood and gathering pollen. If they get caught up on those duties they will be putting away the syrup (instead of just feeding it to the brood) and they will need more cells to put it in and they will build more comb. Right now a lot of them are tied up just making heat for the brood and caring for the brood. They were quick at first because they need comb just to raise any brood.
I wouldn't worry about it. When they need it, they will build it.
You are already doing the only really usful thing, which is to feed them.