Can't seem to get offline transactions to work.

I create the transaction in the online computer using a watch-only wallet made with the public master key, transfer the transaction input file to a USB key, connect it to my offline computer and load it into the real wallet there made from the seed. The window that pops up says the Status is unsigned and that the transaction is unrelated to my wallet. It also displays (correctly) the inputs and outputs. I click Sign and enter my password. but nothing seems to happen, it sends me back to the transaction window. I save the file to the USB key but its identical to the input file, the transaction wasn't signed.

I'm using electrum 1.9.5 portable on both computers