Particle tracking questions

I am trying to run particle tracking example in CE-QUAL-W2. But I got an issue here.

The particles were initialized uniformly at the first segment that has water and were able to transport for a few steps. But after that, the particles were blocked at one segment that is far from the downstream gate. It seems that the particles hit a wall. They were either attached or bounced back to upstream. But the segment is not a wall and has horizontal velocity along it. Can anyone give me a hint of what is going on here?

I am very new to this model and the questions may sound stupid. Your answers are important to me. Thank you in advance. Please let me know if you need any additional info.