Why CST does not simulate long waveguide?

Why CST does not simulate long waveguide?
I am trying to simulate long waveguide and after certain length CST stops calculating s21.
Problem details:
Boundaries - open
Simulation range - 0-300MHz
The structure - waveguide: 3 x 3 m
Waveguide walls made with lossy material 3m thick in every direction.
Two antennas are installed close to the ends of the waveguide.
I use Transient Solver, calculate s-parameters only.
The problem works properly with waveguide length up to 100 m. Simulation is not crazy long - ~ 1 hr. When increase length from 100m to 200m simulation runs, S11 is still calculated but S21 - not
Why this happens? What should I do to circumvent this problem.
PS: I tried other solvers: Asymptotic Solver, TLM Solver - none worked for me.

Hi, I don't know why, but when I needed simulate a long waveguide, due to mesh size, I simulated a fixed length (i.e 2m) and used CST Design Studio to couple it in series the same structure all desired length and calculate the equivalent S-parameters.