Supriyo,
The principle is very simple and you know it too!
If the entry is maintained in T184 (transaction code VOV4), then for that sales doc type, item category group combination - that particular item category can be used in the order (quotation etc.)
In SE16N, table T184, give the input parameters, is AGM maintained?
Further, why do you want to use item category AGM? Why not AGN?
