Ground port in ADS' EM simulation

Dear all,
I'm trying to make a parallel stubs filter. It consists in some parallel microstrip lines finished in ground.
I designed it and then I simulated with ADS. I had no problems until here, the simulation was kind of what I'd expected.
The problem came when I tried the EM simulation. I defined the substrate and I put the pins, but when I run the simulation, the program deletes the ground pins of my circuit, and it simulates as if there was no ground pins.
Lately I tried the same circuit but using coplanar waveguides instead of microstrip lines and I got the same result.

Why isn't my layout reading the ground port?

Thanks!

We can only help if you show more details: substrate definition, pin definition on signal and ground layers. If possible, upload a simplified example workspace to show the issue.