filters don't work after combining several spreadsheets into one

Hi! I could use some help with filters.

I have a spreadsheet I distributed to others to complete and return to me.
Once I got the copies back I combined them into one by copy and pasting the
data into one spreadsheet.

Now, though, when I try to sort a column to exclude all "N"s not all "N"s
are excluded. (data is a variety of yes, y, no, N, yes-dept #'s...)

I've tried removing the filters, and re-filtering, but it doesn't work.

I've tried selecting a cell ("N") that is not excluded but should be, then
filtering on that, but that process misses other information.

Any ideas?

Thanks!