Reward Availability Funny

So, funny thing happening here. Anyone else seen this, or can shed any light?
I've got a SFO-LHR flight coming up, booked as an S, which I'd like to upgrade with miles to Upper.
Usually, the best way to work out if there is G availability is to do a 'dummy' award booking, right through to the point where it asks you for your credit card details to pay the taxes and fees. If there is no G availability on the day you want, it will usually say 'no availability' and offer you a calendar of the nearest available days.
So, I just did this for the date I'm travelling, and I get as far as the stage where you actually pay and confirm, when making the dummy booking.
I phone up VS reservations, and the agent says that there is no availability in G on that flight, which is puzzling, considering I got 99% of the way through the dummy booking. This has happened twice too, once when I made the initial booking, and again since.
I usually make sure I hold onto the web booking session until the reservations agent picks up, at which point I release it (i.e. if there is only 1 G seat available, I'm not holding it online).
So, wondered if anyone else has come across this, or knows a way out rather than just sitting on the phone to reservations an awful lot.
Mike
I've got a SFO-LHR flight coming up, booked as an S, which I'd like to upgrade with miles to Upper.
Usually, the best way to work out if there is G availability is to do a 'dummy' award booking, right through to the point where it asks you for your credit card details to pay the taxes and fees. If there is no G availability on the day you want, it will usually say 'no availability' and offer you a calendar of the nearest available days.
So, I just did this for the date I'm travelling, and I get as far as the stage where you actually pay and confirm, when making the dummy booking.
I phone up VS reservations, and the agent says that there is no availability in G on that flight, which is puzzling, considering I got 99% of the way through the dummy booking. This has happened twice too, once when I made the initial booking, and again since.
I usually make sure I hold onto the web booking session until the reservations agent picks up, at which point I release it (i.e. if there is only 1 G seat available, I'm not holding it online).
So, wondered if anyone else has come across this, or knows a way out rather than just sitting on the phone to reservations an awful lot.
Mike