Reward seat showing available but not actually available to

Long time lurker, first time poster...
Firstly, let me just say as a lurker I have gained some valuable advice on here from a number of you over the last little while so thank you all for your help.
Anyway, point of the first time is post is that I cant see that anyone else has encountered this, or at least I couldn't find it when I searched the board.
OK, so I have an itinerary from London to Barbados return on hold with upgraded seat from economy to upper on the return. I check availability on the return leg and there is plenty of upgrades available for most dates at the time I want to go currently including the day after I currently have on hold.
So, I want to change the reward seat on my hold booking to the following day. I can see it's 'available' using the site's home page checker thing. However when I call up FC they tell me there is no availability on the return on the following day.
Following a bit of to and fro-ing it appears that the web site isn't completely accurate when it comes to reward availability. The guy on the phone told me that they make some reward availability available for the country of origin of the flight only so when I check availability one way it shows available, but when I try to use that availability on my itinerary which originates in the UK I cant have it. It's to do with being fair by allowing residents of the Caribbean opportunity to take those reward seats rather than UK residents grabbing all the upgrades.
I don't doubt the FC agent was telling the truth (and he was as helpful as can be when he's telling you something you don't want to hear) but if this is the case then it invalidates a lot of what I have previously read on here (and been told previously by FC) about how to check for reward seat availability. It essentially means that their system can show you there is a reward seat when there isn't and the only true way to know is by calling them, which is impractical to do on a daily basis when hunting for upgrades.
Has anyone else encountered this before?
Firstly, let me just say as a lurker I have gained some valuable advice on here from a number of you over the last little while so thank you all for your help.

Anyway, point of the first time is post is that I cant see that anyone else has encountered this, or at least I couldn't find it when I searched the board.
OK, so I have an itinerary from London to Barbados return on hold with upgraded seat from economy to upper on the return. I check availability on the return leg and there is plenty of upgrades available for most dates at the time I want to go currently including the day after I currently have on hold.
So, I want to change the reward seat on my hold booking to the following day. I can see it's 'available' using the site's home page checker thing. However when I call up FC they tell me there is no availability on the return on the following day.
Following a bit of to and fro-ing it appears that the web site isn't completely accurate when it comes to reward availability. The guy on the phone told me that they make some reward availability available for the country of origin of the flight only so when I check availability one way it shows available, but when I try to use that availability on my itinerary which originates in the UK I cant have it. It's to do with being fair by allowing residents of the Caribbean opportunity to take those reward seats rather than UK residents grabbing all the upgrades.
I don't doubt the FC agent was telling the truth (and he was as helpful as can be when he's telling you something you don't want to hear) but if this is the case then it invalidates a lot of what I have previously read on here (and been told previously by FC) about how to check for reward seat availability. It essentially means that their system can show you there is a reward seat when there isn't and the only true way to know is by calling them, which is impractical to do on a daily basis when hunting for upgrades.
Has anyone else encountered this before?