need help in designing a report

i want to design a bill report

i have already designed the report (single) in which it genereates & displays a bill per customer

like i select customer 1 & clcik report i get the bill report for that customer i have grouped as per customer for a particular month of the year

similarly i want a report where it will display all the bills one below teh other for all teh customers avaliable for that particualr month of the year

i grouped as per year , month & tehn customer id

in my database i do get all teh customers

but in the report i dont get anything

how do i make a reprot for displaying all the values avaliable in that table