There are a few things that could cause previews to be rebuilt.
1. The file's EXIF Capture Date is in the future
A catalog will compare the source file's EXIF Capture Date to determine if the file is "newer" than the preview file. If your camera's date is set incorrectly (say, for a week in the future), Capture One will see that the image file is more recent, therefore the preview needs to be updated. Setting the camera's date and time correctly will prevent this. If you find that your images do have a capture date in the future, this cannot be changed within Capture One; all source files are read-only in Capture One, and all work is done by proxy. It would be necessary to use another application, such as exifTool, to change this information.
2. The Cache Preview Size is incorrect
In Preferences -> Image, the Cache Preview Size should closely match the width of your widest display. This will ensure optimal preview generation at a Fit zoom level. If it is set too small, the source file will need to be re-parsed to create a preview and show the image on screen; this process is slower than getting it right the first time. If this is too large, the preview will take longer and include excess detail which is not resolvable at a Fit zoom anyway.
Of note, whenever the preview is zoomed past a level of detail that is included in the generated preview, the source file will need to be re-read anyway.
3. The catalog database is corrupt
It is possible that the timestamps of the source file and preview are not saved properly in the cocatalogdb (the catalog database) or cosessiondb (session database). You can resolve this by using File -> Verify Catalog or Session. Choose the offending catalog or session, then choose Verify. If needed, the Repair button will become available after a Verify. Repeat Verify, then Repair until Verify returns a good status.