The exact same thing happened to us, it was absolutely horrible. We introduced a sleep toy, just this soft bunny thing and now if we know we'll be out and about when it's sleep time, we take the toy with us and it makes things much much easier. She generally won't have a great sleep and often she won't sleep at all but she will get the idea it's rest time and will tolerate us putting the hood down and even a cover over the pram and she just has a "rest."