Excise official document number not updating

Hi Dear Gurus,

                        At the time of excise invoice generation document official number is updating with same number for subsequent following documents. For example if you take 10200000020 is number for one excise invoice same number is updating to further documents for that series group, it is not taking numbers like 10200000021, 10200000022.....like that, its only taking single no i.e 10200000020 for all documents which are undergone save. please help me in sort out this issue very urgent ...

Thanks

Srmantha Ragava