Excel autocomplete not working consistently

I am using Excel 2010 to work on a large scale data entry project that involves thousands of different names. All these data are organized into a single column, and there is more often than not a large variety in names as these are from all over the U.S. and Canada.
An example can be seen here:

Bill Leighton, Jr.
Les Siebert
Andrew Kosiski
Tom Svoboda
David Livingston
Andy Wilkinson
Matt Buller
Micah Veleba
Dwayne Pospisil
&&&&&&&&&&&&&&&&&&&&&&&&&
Jase Kaser
Les Siebert
Philip Loseke
Tom Svoboda
Alex Humphrey
Andrew Kosiski
John Bridges
Micah Veleba
Denton Buller
Tom Svoboda

The ampersands represent, in this hypothetical scenario, the line between data entered on Monday (above the ampersands) and those entered on Tuesday (below the ampersands). For some reason, when I enter "Les Seibert" on Monday, I can autocomplete his name by typing (however many letters as necessary; a name like "Les" is very rare in my data) "Les" and the "Siebert" will come up. I can hit Enter and move on with things. However, on Tuesday, when I come across ol' "Les Siebert" autocomplete won't fill in the rest of his name; I have to type the entire "Les Siebert." Yet I will find "Les Siebert" in my list of names when I press Alt + ↓.

But why does autocomplete stop working on data I entered on previous days/session? Why doesn't it remember "Les Siebert" on Tuesday as well as Monday? Is there anyway, without using Macros, to make Excel remember stuff on a day-to-day basis? My other Excel files remember my data, so what could I have done to make this one not work anymore?