Formula Works in one Table, but not Another. Why?

[#1]

I have a formula where I am summing the market potential of a customer for a specific market and then dividing that potential by the total potential of the entire group (Sum(Potential)/Sum({$<Customer=>}Potential) ).  When I do this by market, it works.

Overall     4.7%  (Total line)

Business  6.0%

Education 5.8%

Museum   4.3%

etc...

However, when I change the dimension on the table to County, using the FIPS code, it works on the "total" line, but not on the individual county lines, using the same formula.  Any ideas as to why this would be?

Overall     4.7%  (Total line)

Business  100.0%

Education 100.0%

Museum   100.0%

etc...

[#2]

Sorry guys, the second part, with the FIPS code, would look like this:

Overall     4.7%  (Total line)

06-037     100.0%

06-059     100.0%

06-065     100.0%

etc...