trying to rock her back to sleep
What do you mean by this? Picking her up and rocking her?
Because movement is the most effective way to resettle (ideally with a dummy, but if not then just movement). But this needs to be movement that doesn't disturb baby.
Linking sleep cycles comes through practice. So you expect to have 30-45m naps but every single time, every nap, every day, you attempt to resettle.
Initially most resettles will fail, but when you do get a successful resettle you might need to maintain that resettle technique constantly to keep baby asleep. Then over time more resettles are successful and less work is needed to resettle. Then once baby has mastered linking sleep cycles, you need to do less and less until you don't need to do anything and baby stays asleep throughout the cycle change.
you do the resettle matters
The idea is that baby doesn't wake at all. So baby goes from deep sleep into light sleep, you notice this change and resettle so that instead of going Deep>Light>Awake>Resettle, you go Deep>Light>Resettle>Deep
The "tell" for moving into a light sleep may be tiny, you have to closely watch and pay attention to sleeping baby. It might be a limb moving, face screwing up, hand forming a fist. If baby cries, it's too late since baby is now awake and your chances of resettle are tiny.
you do the resettle matters
Expect to have to work at it. Holding baby's hand is not going to cut it here. You have to actively lull baby back to sleep.
If you're cosleeping (conapping), stick the nipple back into baby's mouth and start rhthymic bum pats. Comfort sucking helps significantly to resettle (why dummies are amazing things).
If not cosleeping, the most independent way yo teach baby to lengthen naps is to go up sleep in something stationary, that moves. For example a bouncy chair, rocker, swing or pram pushed back and forth on the spot. I favour a bouncy chair.
The way baby goes to sleep initially matters
It's essential baby goes in this wide awake and falls asleep while in the bouncer (or whatever). So if baby isn't doing this, it's something to work on as a priority. Baby won't resettle in-situ if he/she never learnt to go to sleep in there initially.
It's then about using just enough movement to get baby from awake, through the light sleep phase and into the deep sleep phase, then stopping. Stationary while deep sleeping. Then the moment you see a grimace or tint bit of responce from baby - restart movement and keep it going.
Movement wants to be even tempo, even ferocity and very rhythmic. This is why a pram walk isn't as effective for this as a pram pushed back and forth on the spot (or a bouncer).
What about cot naps?
It's hard work to teach resettles in the cot. Not impossible, but it's easier to keep all naps in something that moves until baby is consistently linking sleep cycles, and then move them to a cot.
If you insist on cot naps, then napping, white noise and dummy are your best ways to teach linking cycles. You'll also need to ensure you sit right by the cot for all naps do that you can pay close enough attention to notice baby coming out of deep sleep (But before waking up), so you can start patting amd shushing back to sleep.