Sizing problem - Boulevard in baby alpaca

Hi everyone - need some advice on this. (I’ve posted this on the Newer Knitters group on Ravelry, too.) My mum is currently trying to knit herself Boulevard (in Rowan Classic Reminiscence) with Artesano 100% baby alpaca. She has already (very successfully!) knitted it for me in Manos Silk Blend, but her one is causing problems. What we really would like to know ([I]before[/I] she frogs most of it and tries again…) is whether it is simply a matter of adjusting the sizing, or whether this yarn is never going to work for this project. I will now hand over to her to explain the details.

I thought I had satisfactorily got the tension correct but the back is knitting up weirdly - the hips and waist are on the generous side of okay but the bust/underarm would fit a 44" and I’m knitting the 36"!. I’ve never used 100% baby alpaca, and before I start again on a smaller version, can anyone tell me if there is some reason why this is never going to work? Pretty please?

Alpaca stretches, so that may be part of the problem. You can adjust the shaping and decrease sts between the waist and bust.

OK, thank you.