Most likely the foreign keys that link brand to category and description to brand are not included in the applicable recordsets.
For dynamic dropdowns to work there must be a "link" between the recordsets in the form of a foreign key. When the data assist wizard ran did you get just two values in the recordset i.e. the id and one other value which you choose as the text option for the dropdown ? If so you need to open up those recordsets from the servers behaviour panel and add the extra required foreign key that dynamic dropdowns requires.
I have a general tutorial on dynamic dropdowns here multiple-dynamic-dropdowns/ which shows how it all goes together