Is she within hearing distance to you putting your other child to sleep? or is there another noise at 6pm that could be distracting her. I know at my DD gets distracted by the school kids on their way home from school so that ruins my afternoon sleep and everyone in my neighbour hood decides to mow lawns at 6pm. However my DD was always subject to noise early on, we didnt make things quiet but its the sounds she not use to that keeps her up or wakes her up.
The other thing is the temperature change between the day and night, is there a chance she could be feeling a bit warmer/cooler than usual?
I say stick at it, my DD was in a routine from being in hospital for 4 wks and had to self settle that whole time, i bought her home and stuck to what we did in the hospital, once everyone else started interfering and rocky, patting you name it, i had a nightmare on my hands. I bought Tizzies book when she was 8mths, saw where i went wrong, instead of having a baby i couldnt get to sleep by 10pm - Midnight, i now have a happy bubs going to bed at 7pm and falling to sleep in 20mins and staying asleep. Now if anyone rocks her, they get told off, I have bubs no2 on the way, and i want a good sleeper.
Plus i am the worse sleeper ever, since i can remember from a kid i never slept at night. I dont wont this for my daughter so any advice from books or forums is all taken on board.
The books not a bad thing, Tizzie is very opinionated which a lot of people dont like, but its up to the mother to read the queues from her baby and comfort her baby in the way she feels appropriate.
Good luck with trying to get bubs to sleep, pm me if you want to talk or run any ideas past me.