Often when you widen the underarm, it automatically reduces the sleeve cap height. Not good! This just then binds on the top of your shoulder. Typically, you need to widen the sleeve and maintain the cap height. And after the frustrating fixes of slashing and spreading the pattern, this easy technique you'll find with Sure-Fit Designs, adds the width you need and usually you don't do anything with the cap height. Just make sure you've measured the new back and front sleeve cap lengths and compare them to the current armscye lengths. The back sleeve cap should have about 5/8" ease and the front should have about 1/2" ease.
If you add more than about 1" total width to the underarm width, you'll likely need to widen and/or widen and drop the bodice underarm point to accommodate this larger sleeve.
Take a look at this video to see how easy Sure-Fit Designs deals with this issue.