Can't group/ungroup data in pivot table

Dear Sir!
In Excel 97 - i creat a pivot table (3 col.: category,date,amount).
after creating :
1. i grouped the date(in pivot worksheet) col. (by year,mount) .
2. i add new rows
3. return to pivot and refresh data
4. the pivot table now is ungroup
5. i can't grouped it again (error message)

is there any option to do that ?
why after grouping first - can't group again (although trying to ungroup)

thank's
Itzik