plant detetermination in so

Dear Friends,

I have a small query about plant determination in sales order.

I know plant is determined in so from CMR,MMR and CMIR. I have maintained all the three but still when i create the so the plant is not determined in the so and also when in change mode too i dont find the plant if i enter the plant while i craete sales order.

what could be the reason

Any other settings to be done

Thanks

ajay