well, a couple of options come to mind.
1)placement of the formula, if you place it a group header or footer, it will only look at the first or last record, so usually this would be placed in a details section.
2) the formula is not working as expected. I usually will break down a formula to make sure that it is 'seeing' and reacting the way I expect it to. For example: if a formula has an if statement, I will start with just the first if, and let the formula print true or false, just so I can see if it is working, I would also include on the same line of the report, the field(s) the if statement is checking so that I can see that all lines up as I think it should.
HTH