Hi, what age is this for? It says 0-36 months on the website but a 0 year old definitely couldn't fit in here....? I am trying to put my 10 month old in it and it is not working. The straps seem to be for a much bigger child e.g., lots of space between his crotch and the strap. If I tighten the straps around his body then it is too tight on the straps that go horizontal across his tummy and still loose on the upper traps around his shoulders and there is always the space in front of his crotch. Cannot figure it out!
Firstly, the 0+ age allowance is often a faulty one OR one based mostly only on the fact that the pushchair is either compatible with a carrycot or a newborn nest/cocoon, or based on the lie-flat possibility. Here, as the seat is actually a rather large one, I would suggest a nest insert, if you did put the harness straps at the lowest possible and the tightening doesn't help.
About the nest, try something in the likes of https://www.amazon.co.uk/Newborn-Support-Cushion-Toddler-Strollers/dp/B07JMJSFX2/ref=sr_1_5?dchild=1&keywords=stroller+insert+newborn&qid=1629032307&s=baby&sr=1-5 or https://www.amazon.co.uk/KAKIBLIN-Stroller-Support-Carseat-Cushion/dp/B08BCRQTL5/ref=sr_1_2_sspa?dchild=1&keywords=stroller+insert+newborn&qid=1629032307&s=baby&sr=1-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzQ1NLOFVMVVZYVFFZJmVuY3J5cHRlZElkPUEwMjQzNTY5MlNROUJIS0VRSFpCQSZlbmNyeXB0ZWRBZElkPUEwNjcyMzY4MjVUUDVETFc4MVozSSZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=. These help to reduce the amount of space in between.
Not much else to do... just simply hold on, in a few weeks, the baby will grow into the space.