The other thing I can think is: did you notice some difference in the day when she takes so long to settle? Maybe some activity the os her, like swimming or playgroup or shorter nap, so in this case 6 h A time is too long? Or, on the opposite, some too less activity that days so she is not tired enough to switch off at bt?
Another thing: it should be more and less at 18 mo when we changed bt routine and put the bath before dinner instead of before bt. This was because we noticed that a late bath os F and he ended up crying for tiredness while he came out from the tube. Doing it before, with him hungry so more willing to come out, made the trick. Maybe worth a try too.
