Did you ask them to try?

I've secured bassinet seats 3-4 times calling, initially some have said they're unavailable, on asking them to try they have gone, 'Oh yes, its worked'

So unless they've said policy has changed (doubtful) then I would give it another go with another agent, saying you've done it before, asking politely for them to give it a go.

Remember that anyone connecting to your flight will have access to seats from when check in opens for their initial flight, so prime seats may well be snapped up by the time you get on at T-24