Ion Firmware update - console tells me to update but the update button is greyed out

I've just updated 4 of our Ions (non Xe) to 2.7.3. All gone fine on 2 of them, both of which asked me to update the firmware and that all went fine.

1 of them did not ask me to update the firmware, and is on an older firmware.

1 of them keeps asking me to update the firmware, but when I go to the maintenance tab and select the entry in red the update button is greyed out and not available?

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