update data problem

I am trying to update about 25 rows in my database with data that is generally found in a lookup field. When trying to do this using the edit function in the view it accepts a change and then when I attempt to change the next record with the same value it accepts the change and removes the value from the previous record. This will continue if I attempt to make the change again. I have seen this before and worked around it but have hit an issue here and so need a response.

I also attempted to make a bulk edit which also failed

To try and work around this I exported the rows to a spreadsheet and thought I'd attempt to make the change using the excel migration tool. That also failed to make the updates.

Thanks
Simon