What Haddon said can help to encourage them to build new comb as well as straight comb in a top bar as well by placing empty bars between full ones.
The first year, especially housing, a TBH is the most difficult. the second year will be a lot easier for you. Remember that producing comb takes a lot of energy so feed, feed, and then feed some more.
In my hives, with the exception of heavy flows that may skew the housekeeping I can almost always expect to see a (listed from the front entrance to back) a bar or 2 of pollen, brood, long term honey stores in the back. How many bars of what can vary greatly through out the year but it will mostly be in that order unless a flow is real strong and the house bees don't have time to organize it, a heavy brood laying cycle like the spring build up may skew the order as well. As long as you make sure they have room to build being honey bound is usually not a long term issue in a top bar as they will just build more bars.
I hope this helps.