Hi,
I have to agree with Gabriela, it seems that you are entering the transaction me21n BEFORE you are trying to copy the values.
Can we assume that the custom fields where you are copying the values from are not in transaction me21n and that you have a button you press from another transaction to get there? If so, like Gabriela stated you will need to have the copy value before the transaction is entered like so:
The same would also apply to material and quantity.
If this is not the case, perhaps a screenshot of your flavor would help?
Ian