Be sure the cursor is on the row and column you want to be considered headers. If headers are in row 3, and you are in row 1, JAWS thinks row 1 is headers.
My preferred way of setting row/column titles is to use Excel's bookmarking feature.
1. Place the cursor at the intersection of the row and column that are the headers.
2. Press the applications key.
3. Arro to "define name dot dot dot" and press enter.
4. Type the word title and press enter.
If row/column titles are not spoken, alt+tab away from the workbook and then alt+tab back.
A cool thing is that if this is a worksheet you use repeatedly--as for example a sheet for January, a similar sheet for February, one for March--save-as will include the bookmarks (row and column titles) on the new sheet.
This is explained in the JAWS training materials probably more clearly than I just did it.