I have an Isofix base for my Britax baby safe plus SHR. When the base is in the car with the seat attached, the car seat seems to sit up quite a lot? Like she's sitting up quite a lot. So when she falls asleep, her head flops forward. I don't like it at all. I bought the base when she was two months old, her head flopped everywhere when we first had it so I didn't use it. I've only just tried it again. Does anyone else have this problem? I've taken it to a car seat centre and they say it's all installed properly and they've never heard of the problem.. Anyone else have this?