R_Vera:
What video format are you capturing?
What are your computer specs?
..................and - Welcome to the Forum 
1) Do the Source Video Thumbnails show in the Album & Timeline, as per my Screenshot.of S12 Sample Video.
2) Test with S15's Sample Video. Should be in your Shared Videos Folder.
3) Post a screenshot of your problem. How to do that; HERE.
My Screenshot: