Knitting help please? Armholes too snug on cardi!

[iknowimcoming]

I knitted myself a cardi ages ago but haven't finished it as when I've sewn it together the armhole seam is a bit too tight I tried unpicking and using a 'stretchy cast-off' but still too tight, any suggestions please? Should I unpick and increase the stitches on the last few rows maybe? I'm not a very experienced knitter sadly!

Bump?

is the seam too tight on the piece that goes on your arm, or the piece that goes on your body? or both?

Apologies @parietal minor disaster with a houseguest, now resolved! The body and arm holes are fine until I sew them together so presumably I need a stretchier stitch than mattress stitch to sew them together? Or maybe I'm doing my mattress stitch wrong?