Why are the gridlines in one of my docs off?

We need more to work with.

"Off" in what way?

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Can you provide us with a "sanitized" small example file?  Include text in the example file pointing out exactly what we are supposed to be looking at.

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As well, (in addition to, not just instead of) you could provide us with screen captures here of a "good" and "bad" grid.

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Off the top of my head, when I single file is having  any sort of problem my first suspicion is "document corruption.  The first thing I would try to do is create a new copy of the file.  Open a new blank file. Add a couple of blank paragraphs at the bottom of the original document. Select all of the document EXCEPT those blank paragraphs. Copy and Paste into the new blank document. Save with a new name.

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You don't want to copy/paste the very last paragraph mark in a document because it is the last "section" in the document and actually "contains" the document formatting information. This last paragraph mark may contain the corruption, which is why you don't want to copy it.

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Trouble shooting problems in files can be like a visit to the dentist, a long, slow painful process of us trying to “extract” the clues needed to recognize the problem in a back and forth flow of questions and answers so that we can come up with a / “the” solution.

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Often it is faster and easier for everyone if we have a “sample file” get "hands on", to look at, and to “play with”.

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This next link provides some tips on setting up a sample file and specific instructions for uploading and "sharing" it for us to access:

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https://answers.microsoft.com/en-us/windows/forum/windows_other-winapps/trouble-shooting-share-onedrive-file/a231a097-bcbf-4e34-ad6c-a33118baf471

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This article describes another way of sharing Office files:

2020 03 09- Share a cloud stored document from Microsoft Office
https://office-watch.com/2020/share-cloud-stored-document-microsoft-office/
Using “Invites” to specific people from inside Office apps. Modify document access permissions and add a “note” with “how to” instructions for recipient

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The first 2 minutes of this video gives an example of What I mean by a “simple” example. Use short simple names and quantities, and just 2 or 3 rows per sample data you want calculations done on.  You want simple numbers so you can do the math checks in your head.

Col- Stacking Columns of Data (Unpivot, Split Columns by delimiter, and everything!)(PowerQuery)               2017 04 16
http://ozdusoleil.com/2017/04/16/power-query-stacking-columns-of-data-unpivot-split-columns-by-delimiter-and-everything/
We’ve got headers and a large number of paired columns that need to be condensed down to just 2 columns. 

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