If it appears that Prep Fields and Tables is not actually creating the resulting fields/tables, check in %temp%\Narrative1. Normally files should all be placed in subfolders there, named for the Excel file. If the files are dumped directly into the folder instead, this may be because N1 is operating based on much older code. This can happen because the Excel file is on OneDrive, which causes the Excel program to, in limited ways, assume N1 is on OneDrive as well instead of reading the code from Program Files.
Turning off the Office integration in OneDrive can sometimes fix this.
Right-click the OneDrive system tray icon, click Settings, click the Office tab, and un-check "Use Office 2016 to sync Office files that I open".
Storing files locally or using other file hosting solutions can also rectify this. This can also be caused by an N1 file being saved as .xltm instead of .xls or .xlsm. Re-save it as .xlsm, close, and re-open if that's the case.
Another possible cause for some tables failing to prep is external links in the Excel file. Go to the Formulas ribbon>Name Manager in Excel, and group by "Refers to" to see if there are any large blocks of external links. These can most likely be deleted en masse, which can alleviate this issue.