This isn't just upgrade availability.

I've had the same problem with buying tickets.

"There's no space at all on flight X"
"I can buy it online"
"The system says there's no space"

So I buy a refundable fare, give them the booking reference, they spend half an hour looking at it, cancel the refundable one I bought, come back, and say "There's no space".

But I can still buy it on the website.