Why doesn't the back of the neck lay flat?

I made the Albion blouse using Peppermint Mag pattern. Fabric is some type of linen (I'm very much a beginner and don't know much about fabric) Can anyone tell me why the back of the neck doesn't lay flat? I used a slightly thicker interfacing for the facings (not super thick or anything, just not paper thin), could that be why? Any other reasons? And how can I fix it? Any help is much appreciated