K, sounds like you have to ease the cap of the sleeve into the hole.
I'll try to explain this as best as I can as it's not like regular just straight edge to edge kinda thing...
First, fold the sleeve in half in the middle of the cap and the underarm, now take some stitch markers and place at those spots. Now, take those markers and match them up with the bodice seam and the shoulder seam, pin those in place (K sorry should mention here that pieces should be right sides together) Now that you have the centers matched up, you will match straight (no easing or gathering or stretching) from the underarm stitch to the point where you start the cap of the sleeve. Do that on both sides of the underarm center. Now comes the fun part. Easing the cap into the space allowed by the armhole, easing or squishing together the cap as you go, be sure to pin together. Do this on the other side of the top center of the cap. Now that you have it all pinned, you can now sew (with yarn of course) together, starting at the underarm center around to where you started.
Dont' worry about it looking all puffy. Once you've gotten it all eased in it'll sit on your shoulders much better than a plain straight to straight seam.
Hopefully this helps hun. I tried the best I could without pics.