"We're sorry the option to upgrade is no longer available."

Hi V-flyers
I appreciate I could ring FC but don't want to be on hold for ages
Situation - I have a LGW>CUN>LGW booking in PE(H).
On 12 March I can see the return has UC availability in Z+, and indeed I get offer in my booking to upgrade this leg.
All fine.
I click 'calculate total' button, and then get this:
"We're sorry the option to upgrade is no longer available."
Any ideas why? (Given the availability is still there)
VS IT glitch?
cheers
I appreciate I could ring FC but don't want to be on hold for ages

Situation - I have a LGW>CUN>LGW booking in PE(H).
On 12 March I can see the return has UC availability in Z+, and indeed I get offer in my booking to upgrade this leg.
All fine.
I click 'calculate total' button, and then get this:
"We're sorry the option to upgrade is no longer available."
Any ideas why? (Given the availability is still there)
VS IT glitch?
cheers