I have three tbhs, two were started from scratch the third had comb from the first two to get them started. I guess I was lucky and one built straight right away and the other I corrected quickly as the package started building comb, never letting them get too far off. It did slow them down as their first couple weeks of comb was thrown away, or mangled as I "bent" it back into shape. I use 1/4" square stock for starter strips, maybe the string isn't enough? FYI that hive is in it's third year has requeened itself at least twice and is extremely strong, so the early set back did not doom them.
This year I am planning to try some lang type hives. I have been turning empty top bars upside down and rubber banding a foundationless frame to the bar and putting it back in for them to build on. If you did that with foundation in the frame, that may be easier and give them more of a guide than an empty frame top bar.
Of course I built a square tbh (TTBH?) based on lang widths, not a Kenyian style. Are your sides sloped?