> I just checked if there are keywords in the properties but there are no keywords.
That's too bad!
When you look in the folder tree, do you see any grayed-out folders, or drives that have an exclamation mark in their icon? If so, it could be that a folder or drive has been renamed from outside TP7, and ThumbsPlus no longer recognizes it. When that happens, the old thumbnail records (that have keywords) can no longer be matched to the image files. Instead, the image files will then appear under a different drive or folder name and their new thumbnails usually won't have keywords.
Keywords can also be stored inside of image files (in their IPTC/EXIF header). If that is the case for your images, ThumbsPlus can copy those keywords back into the database when it makes or updates thumbnails. For that to work, you have to go to Options | Preferences | Keywords and turn 'Automatically generate keywords from: IPTC Keywords' on. Then select the thumbnails and press F3 to re-make them. But again, that only works if the keywords are stored inside the images themselves.