Not sure what route you are flying on, but many of the Virgin flights, especially those to MCO (and the one I'm on next year from MAN to St Lucia) - the system has wiped all seat reservations from those ever so popular 2 sets of seats at the back of the plane in economy, and these are now effectively blocked from booking. Occasionally it is still letting you book those in row 61 though, which is the first set behind the sets of 3s at the back.
There has been alsorts of speculation as to why Virgin have done this; from changes in aircraft configuration, to charging extra for these seats !
However, from reading through (the extensive) posts on other boards, it would seem that even Virgin call centre staff are unclear why this has occurred (citing numberous reasons to different callers). However, most people have reported as being told that these seats at the back are now only being allocated on checkin for 'operational reasons'.
A minority of people have also reported seats near the front of the plane have also been 'blocked' too and had their pre-allocations thrown out.
Might be worth checking your seats if you are flying with Virgin, and the seat location is important to you and
especially if you have 'bagged' any of the sets of 2 seats at the back of the plane.
If anyone with some insider information into the workings of Virgin could enlighten us exactly what is actually going on with the the "back seats in 2s" situation I'd be interested to hear.
There is not much you can do if your allocation has been changed since Virgin reserve the right to change them, and they are only considered to be a 'request'.
PS Since my seats all got thrown out too I rang up to get them re-assigned to be told that the outer seats in rows 45 & 46 are effectively now in twos as there is a 'block on the 3rd seat....don't know how true this is (I won't know till next Feb !) - but may be useful to someone ?