Here is a useful tool for figuring out why your Form Sets aren't working the way they should. It is the first and easiest programmatic step after you've eliminated a Path issue as the problem.

Another “first step” is to open the folder that is the source for all the Forms contained in the Set. Inspect EVERY item in the folder. Is each and every one a Word document? If not, any PDF, graphic, or any other type will cause a Sets failure, guaranteed.

The most extensive diagnostic tool is to divide and conquer. Select one half the forms in the Set to Fill and test them. If it fails, divide that group by half again and test it. Continue until you've found the culprit. If hone fail, repeat the process with the second half of the folder's contents. You'll want to do this using Sets' tools as opposed to taking forms out of the folder and testing them one at a time as there is potential inter-play between forms in Sets that may influence the outcome.

However, also be aware that in an update in the late summer of 2022 Microsoft introduced a bug in a Word update that interferes with the operation of the Sets feature.

If your Sets once worked but now do not, perhaps allowing only 1-2 forms at a time or none at all, the Microsoft bug is a good suspect.

Here are two articles that address disparate causes, one of which will solve your problem

When a Set fails, a Microsoft Word Bug May Be the Cause https://kb.theformtool.com/202698186-What-to-do-when-things-turn-hot/360049245693-When-a-Set-fails-a-Microsoft-Word-Bug-May-Be-the-Cause?from_search=108090938

and A Bug From Microsoft, Build 20088 at https://theformtool.helpjuice.com/admin/questions/1556869