There have been messages here in the past in which a limit on the # of items
in the drop down has been discussed. I seem to recall that the # of unique
items that can be presented is capped at 1,000. If what I recall is correct,
it seems likely you are trying to find something that is dropped out due to
the limitation.

"Stealy Dan 1" wrote:

I have frequently used the Auto Filter feature with my spreadsheets.
Recently I clicked the down arrow to set the criteria I wanted to use for the
filter, but the data (a name) was not on the list for me to select. Any
ideas why, or what I should try?