Me and my friend have completely different kids so I will explain both the scenarios and what we did
Olivia: Huge panic artist mother had her in a safety sleep from about 5 months due to her rolling around the cot, I didn't want her to bump her fontanel. She had always grizzled to sleep and when it came to the safety sleep needing to come off the bed (it was becoming a restraint rather than an enabler), we had to go through a weaning process of about 4 nights of hell and 2 weeks of general unsettledness. We hit fever pitch with going to sleep so I started putting her down with a bottle which hence forth solved all problems and she has been a kick arse sleeper ever since.
Casey: Cruisy mother who never bothered with a safety sleep as it made her scream blue murder, usually let her grizzle to sleep as well. Casey hit 14 months and started staying awake in bed so all stimulation was taken down from the side of the cot. Things progressively got worse and she is at 20 months now and in a big bed with the door shut because she managed to start scaling the cot and just climbing out of bed.
I personally think it is different kids and there is a lot of major factors that go along with that. Casey is getting closer to the 2 year age and is sleeping less and less and will just probably be one of those kids that gets enough sleep at night (she sleeps really well at night) than during the day. Olivia isn't the greatest sleeper at night and usually loves going to bed to have a bottle, she will probably carry on her naps for a while yet.
My suggestion would be to see if you can keep her moving for a little bit longer, on top of her just being a type of kid, maybe she could be displaying signs of boredom. Olivia has needed a lot more stimulation when she dropped down to one sleep a day as well, this is when I bumped up activities like going to the playground and enrolling in coffee group for extra stimulation. I also stick to the same bed times, she is in bed by 7.30-8pm at night and sleeping for no more than 12hrs and 12-12.30pm in the afternoon and no longer than 2.5hrs for a nap. Sounds rather anal but it stops sleep cutting into other sleep and sticks to a routine that she is stuck in.
A long ramble but I hope it helps some.