Please check whether shipping tab is activated at STO item level. This is once of the primary reasons for your issue. If shipping tab is not activated in your STO then check following points.
1. Set up for STO
2.Receiving Plant as a customer & sending plant as a vendor
3.Sales area of this customer & sales area in material master should be common.
4.Transportation group & loading group in material master
5. Vendor code in customer master & customer code in vendor master.