mail merge - header data error

I am in the process of a mail merge with a pages document and a spreadsheet in Numbers. I was able to successfully do this with the same files last week. This week, doing the exact same thing, I am getting an error message informing me that I need to use a Numbers doc with headers and data.. I have tried to see what the problem with the file is and can't figure it out. There are headers and plenty of rows of data.. please help..