Your question is very generic, and there are many possible mistakes in using EM tools. Use a mesh that is fine enough where it matters, and make sure your ports (including ground ref) are defined properly.
In recent ADS releases, there are different emSetup templates for different applications. Settings for a 5 GHz PCB are obviously different from a 200 GHz on-chip antenna simulation.
It is a good idea to look at current densities from simulation, this might show if mesh is fine enough.
You might also want to check if Keysight offer Momentum trainings these days (possibly online?)