Purchasing UOM not reflecting from ASL

HI All,

1) UOM definitions details,

Class: Measures

UOM: SH (Base Unit of measure)

UOM: RL

UOM: CT

2) At Item level;

Item: 9000000000

Primary UOM: SH

Tracking: Primary & secondary

Pricing: Primary

Secondary: RL

Defaulting: Fixed

3) At ASL level;

Purchase UOM set as CT

Sourcing rules has been set.

After PO auto creation the P.UOM is SH instead it should reflect CT which is set at ASL. ? Is there any other setup required to achieve this?

Thanks.