Sort Error

I'm using Excel 97 SR-2 and I have a strange sorting error.

I attached a small portion of the 4000+ rows db I'm working with.
It is currently sorted on col A (no problem).

But when you sort it on col AA, some rows with AA content end up sorted above the rows with empty AA (contains a space) and some end up below.

I've tried to reformat the AA col to text and others types and I still can't get it to sort on col AA properly.

The one difference I found is that the AA content of some rows was copied from another .xls file cell and some were keyed into the AA cell directly.
To see what I mean: sort the file on AA then key 22533 into the col AA cell that currently contains 22533, sort again on AA. The subject row will move to the other group.

How do I fix this so the file will sort properly.