document sequence number is not generated

Hi,
I am trying to create chargebacks from trade management and document seq number is not generated, so I have done the following steps:

1) Enable the Sequential Numbering Profile Option
2) Enable the AR: Document Number Generation Level Profile Option
3) define chargeback trx type and assign it in trade management
4) Define Sequences
5) Define Categories
6) Assign Sequences to trx type

I also assigned a document seq for other trx types that is working to the chargeback trx type and still it is not working and it is working in other instances !

please help,

Thanks,

Ouafaa