Hi,
yes, the operations you mentioned (in particular the trajectory snapshot structure), allow the extraction of one single snapshot. If the snapshot given as a result is always the same, it could be that the input trajectory only has one snapshot. Try to download the trajectory and explore that. Was the trajectory generated with an MDWeb setup workflow? In this case, it can have just one snapshot, as these workflows were designed to prepare a system to be used as an input for a MD simulation, but not to run a production MD. More information in this thread: Limited Molecular Dynamics Simulation Time.
I would also recommend you to take a look at the BioExcel Building Blocks (biobb) website and its associated BioBB-Wfs server (Homepage - BioExcel Building Blocks Workflows)), which are more recent tools that can help you on the MD setup and run processes.
Thanks for your interest in our tools!
Regards,
-Adam-