In my experience it seems that's just how they do. I've had a TBH in operation for going on 4 years now, actually the same colony. They were originally in a TBH made from purchased plans with an inner length of only 27 inches. The 1st year they filed it up more than 2/3 as I recall. The 2nd year I was afraid it might be not long enough, so I built another one same specs but just doubled the length and transferred them over. It's now about 2/3 full again from the end of last year. This year the 4th year they have not built anymore comb. It seems they might have now maxed out and built all the comb they ever will for the size of colony they will ever be. They therefore might not ever fill this double length TBH to it's full capacity. They stop building comb every year around early June at latest.
BTW, I suggest having bushes or small trees near the front of your TBH that are low to the ground making it easy to catch swarms. If yours is like mine, every between late April and throughout May it will spit out a couple swarms. Mine did the very first year just about a month and half after I installed the package into the hive.