I've always had trouble easing a full sleeve into the armhole. Recently I made a jacket where the instructions said to ease the sleeve cap by using a strip of fleece. You stretch the strip as you zig-zag it close to the sleeve cap edge. I tried this & it worked great!
Now I'm working on a blouse that has a full sleeve that has to be eased. I first tried the typical 2 rows of basting stitches, but ended up with puckers. So I cut a 1/2 wide strip of a stretchy knit & zig-zaged it to the sleeve cap edge. It pulled it up enough that I was able to sew the sleeve in with no puckers!