Hi Zahir.
I haven't had experience with static sync while leaving asset in situ, but I do use static sync as an ingest method. When I was having some issues generating previews, I looked at my previously setup debug log (see RS knowledgebase) and filtered out the ffmpeg lines for at least the command being used. I could then run this on the cmdline against my asset in the filestore, or in your case, I assume against the foreign asset. Hope that offers a starting point to uncover some hints on where the process may be letting you down.
Cheers
Jeff.