Multiple PO's per material group for same vendor in ME59 / ME59N

HI ,

We have following issue

We create PO's automatically from PR's using ME59 .

PR's are Stock Transport Requisitions for intercompany STO.

in ME59 , options active are Per Plant / Per Vendor Sub Range( Vendor sub range is active but no values maintained )

When we create a PO , then system does not consolidate all items in single PO even if Plant is same . It actually creates a different PO based on Material Group in a line item .

It groups all line items with similar material group in single PO .

What is the issue with material group .

is it not possible to have different line items with different material groups in a single PO in above scenario.

Pls. help.

Thanks ,

Prasad