Comparing two sets of data in the same worksheet

I have an excel 2007 spreadsheet that has last month's data and this month's data.

Last month's serial numbers are posted in Column A, this month's SNs are in Column X. Other columns contain names, addresses, etc... Serial numbers are unique by person.

I've tried a couple of formulas, but don't seem to get any errors or answers -- the cell where formula is entered just shows the formula I typed in.

Formulas I've tried so far:

=ISNA(MATCH(A2,X2:X404,TRUE))

=VLOOKUP(A2,A2:X404,24,FALSE)

I feel like I must be missing something really obvious... Column A has data in rows 2-378, column X has data in rows 2-402.

I'd like to know what's in column A that is NOT in column X and vice versa.

Thanks for your help!

I tried absoluting the ranges using $, also tried the =countif function.

Still not working... cells only show the formula as typed in. At this point I'd be happy if I even got an error message, but the formulas seem to be treated as just text.

Really dumb question -- is there something I need to do to enable these forumlas??

Or should cells be formatted any special way?

Thanks!

PS Getting frustrated because I could have done this manually by now, but I know this will come up again!

Thanks to all for the help!

I had to reformat the columns as ChemistB suggested and also absolute the ranges in your functions using $.

Now it works!