The reason they can't promise that you'll be sitting together is because the basinets only fit onto the space in front of the bulkhead. So if there are 2 (or 3) babies on the flight who have all requested bassinets, they have to sit those mums and babies in the bulkhead row. (so in practice you would probably be breastfeeding next to another mum) This happened to us, where there were 4 seats, me dh, plus baby, another mum plus baby and then a tall man who had requested bulkhead for the leg room. Fortunately the flight was full, the man took one look at 2 small babies and moved seats, and then they found 2 free seats together for the other mum. If the flight is full however, you can be stuffed. Bulkhead seats are often requested by tall people, and they are often reluctant to move. The babies always have priority for the bassinets, but the rest of the family doesn't.
As others have said, if you check in early you stand more chance of getting your seat. If it looks dodgy, opt for Dh in the seats across the aisle, so you are affectively together (although, sometimes the seats across from bulkhead are emergency exit seats with lots of leg room, so also popular)
We have flown lots with ds, on different airlines, and mostly they bent over backwards to make th mums and babies comfortable, we always seem to end up with a spare seat between us etc. I think if the babies are happy, the rest of the flight is happy, as no-one wants to sit next to a crying baby!
Hope that Helps
PS, if you can't get seats together, get you dh to remain at check-in and FUSS firmly but consistently until a manager appears and you get what you want. (stay calm, but be VERY persistant) they hate people making a fuss at check-in desks.