plant defauls - me21n

hi,

while creating the PO through ME21N, plant field is getting grey(display only) if we enter any field before entering plant data.

i checked the same in define screen layout of PO for NBF and with transaction codes, me21,me21n,me22n...etc,

i can mention the plant in ME21n, before entering any field, but if i enterd any field(except plant), later it is not allowing to put the plant.

can any body help