Data Validation drop-down buttons keep disappearing

A colleague has a worksheet with data validation lists/drop-down buttons.  Someone who was working with her on the project asked me how to keep pasting the validation down for every day for the year so he wouldn't have to manually do the data validation for every cell.  I had him Copy and then Paste Special and choose Validation.  Ok, so it was working fine for a while.

Last week the colleague contacted me to say that when others went into the worksheet, they were unable to click on a day and get a drop-down after a certain date.  I had her Paste Special and choose Validation again, and also had her copy the entire worksheet, but both times the drop-down buttons would disappear again after that one certain date.

The worksheet is shared.  Could this be contributing to the problem?  If not, why do the drop-down buttons keep disappearing?  We're both very frustrated.  :)
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Rory ArchibaldCommented:
Shared workbooks are inherently prone to corruption but you should be able to apply data validation to an entire column at once without having to do it cell by cell
BloodCenterAuthor Commented:
Well, the problem is that the rows are: day, code, diagnosis, location, etc.  After about six rows of that, it starts over again for every day of the year.  So I have to cut and paste an entire day and paste it as validation.  Furthermore, there are about 20 columns of employees who choose from these shifts, locations, diagnoses, etc.
Rory ArchibaldCommented:
Then you may just be running into another issue with shared workbooks. I'd need to see the workbook to be sure ( please remove any sensitive data if you are going tonpost it)
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BloodCenterAuthor Commented:

Actually, we tried it on a copy we made that isn't shared.  Yesterday I did the Copy/Paste Special/Validation thing for all of the cells, and today none of the drop-down buttons appear.  So now apparently it's not a sharing issue.

I'm so frustrated I could scream.  Why are the drop-down buttons not appearing when I click on the cells, even though yesterday they were?  Again, this is only for cells in which I pasted the validation from rows that had been completed by manually creating validation for each one.
BloodCenterAuthor Commented:
I think I figured out how to fix it.  I went to a cell that had a proper working drop-down and then went into Data Validation, UNchecked the In Cell Dropdown checkbox, and then re-checked it.  I then did that for one set, copied them, and then used Paste Special and Validation, and now the drop-downs are still there when I close out of the workbook and go back into it.  It seems odd, though, that I'd have to do that.

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