I would guess you are entering your summer nectar dearth, and that the bees will not draw any more comb anywhere unless you feed. This is typical behavior, and the reason I'm feeding my package again even though they aren't really taking all that much syrup. They are still drawing comb and I want them to finish out the top box, currently about half depth on all ten frames. The ten or so pounds of sugar it's going to take is nothing compared to not having enough drawn comb to store up for winter, as they can do that unaided this fall on asters and soybeans.
What I've been doing to decide when to add another box of empty foundation on a new hive is to wait until there is a full set of capped brood and the bees are using all but the outermost frames. I usually wait until they have drawn those unless there is a strong flow on and they are likely to run out of room too soon. Once that brood starts to emerge I put the new box on, that puts the new bees to work drawing comb at once, but does not leave a fairly small number of bees in a large space.
I have learned the hard way here that anything not drawn by the end of the spring flow, which is tyipcally mid-June, will NOT be drawn out by a new package without considerable beekeeper intervention. Much better to keep feeding all through the summer to keep them going until I have a winter configuration (two deeps with the top one full of honey and pollen or a deep and two mediums, with the mediums full of stores). With Italians, it's also necessary to feed in August and September, as they usually fail to shut down brooding in the dearth and end up eating all the stores and I have very light hives in late September. I'd rather feed a smaller amount over the summer than have to stuff them in October!