Both ofd my DDs would only sleep when in motion as small babies - sling or pushchair. I walked endlessly, it was frikking exhausting. Both of them, without my help, got better at sleeping without motion by 4-5 months. Everyone says babies are amazing sleepers, but that was NOT my experience. In my experience toddlers are the sleepers ; )
DD2 I literally couldn't put down for the first 2 months. She is now 18 months, and when it's time for her nap I take her upstairs, pop her in her cot, say 'night night' and turn off the light. She wakes up 1.5 hours later.
Anyway the point is, bear with it. Babies find it hard to transition between light and deep sleep (which happens every 40 mins or so) and many, many of them wake up during this transition, which is upsetting for them as they are shattered and want to go back to sleep. If they are being pushed, the motion lulls them back without them having to do it themselves. At about 12 weeks I would put both of mine down for the first 40 mins, then walk them for another 40 mins when they woke up during that first transition. Eventually they stopped waking mid-nap and just went through. I stressed about it endlessly with DD1, just accepted it with DD2, and both became excellent nappers.
we don't talk about DD2's night sleep