How big is he? A 9mo should easily fit into the Cloud T for a while yet.
I do find the Joie inserts tricky, and I think the 0-12 years seats (from any brand) can be even more difficult to get a good fit in.
At 9mo unless he's very big for his age, usually you'll need the wedge and body insert in, sometimes just the wedge. You will almost certainly need the wedge, though, to provide the extra recline, especially if his head flops without it.
Have you moved the headrest up to accommodate his height? You sort of have to shift them up further in the seat in order to get the buckle done up at that point.
I would be tempted to go back to Mamas and Papas and say you're struggling with it and see if they will help you adjust it and if still having difficulty, look to swap it, probably for the Maxi Cosi Quartz, Maxi Cosi Mica Pro Eco or Cybex Sirona Gi i-size, which doesn't go on your base (it has its own base) but it is cheaper than the one which does for some reason.
I expect they won't be keen to swap it because car seats are usually tricky to return, but they might do it on a customer service basis. You can only ask, anyway.
Ideally if your car is sloped enough for head flop to be a problem you'd want the Besafe izi Turn instead, or one of the Swedish type ERF seats like Axkid or the Britax Max Space Pro - but Mamas and Papas don't stock these 