seat map withdrawn for Jan 16 flight?

ok so I messed up and booked the wrong date home for our MCO to MAN flight in Jan. I had booked months ago and had chosen seats etc. We are flying LHR to MIA out (UC upgraded with miles) and MCO to MAN back (Amex upgrade to PE) but I was a day out on our return trip and only realised yesterday.
So I duly rang up, paid my £100 each to change our flight home and waited until it showed on my account. Then I went online to book our seats (had forgotten to do this when on the phone as still a bit angry with myself and just relieved to get two PEs again) but the seat map is all grey .
I rang FC and they said it has been withdrawn, which is normal practice for some routes , esp MCO which are popular. We are flying Jan 30 so not during UK school holidays.
I rang again about 6 hrs later to get a different operator but they said the same thing.
So now really miffed with myself. Have been told I should be able to book seats when checking in online or at the airport.
Anyone experience this before ? Can't help thinking there is something amiss going on here.
At least it is less than 8 hrs flight and it is isn't the end of the world to not sit with Hubbs.

So I duly rang up, paid my £100 each to change our flight home and waited until it showed on my account. Then I went online to book our seats (had forgotten to do this when on the phone as still a bit angry with myself and just relieved to get two PEs again) but the seat map is all grey .
I rang FC and they said it has been withdrawn, which is normal practice for some routes , esp MCO which are popular. We are flying Jan 30 so not during UK school holidays.
I rang again about 6 hrs later to get a different operator but they said the same thing.
So now really miffed with myself. Have been told I should be able to book seats when checking in online or at the airport.
Anyone experience this before ? Can't help thinking there is something amiss going on here.
At least it is less than 8 hrs flight and it is isn't the end of the world to not sit with Hubbs.
