It seems to me that you have to put a follower board,or more simply said you need to gather your bees in a small area something like a area of one comb or two maybee.Thats something no one mentioned to me when I started with my tbh.I f you leave them to much area they will get a bit confused and disorganised especialy when they are starting a new hive,without comb brood etc.They are building it in the back of the hive maybe because it is the warmest area in your hive,also you have to check is your hive nivelated.