The seat map is not available at this time..

So I booked my flights online London to New York (well Newark) but when I goto edit my flights I cannot choose seating on either outbound or inbound flights.
The message goes like this:

We're sorry. The seat map is not available at this time. Your seats will be assigned at check in.

I wondered if this was because I had booked for an infant? or is it just this flight affected?
I have never had any problem choosing seats previously when flying without an infant?

Thanks

[preiffer]

It's also possible that more than 60% of seats are already allocated, and therefore you can't choose your seat.

OR, they've just made an aircraft change.