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You have to use the numbers on the numeric keypad. If you use the ones above the alphabet it won’t work. For an umlaut capital Ü is ALT+0220, lower case ü is ALT+0252. Again, hold down the ALT key and enter the numbers using the numeric keypad or it won’t work. It might be different if you are using Outlook; that might use the same shortcuts as Word.
I forgot to include a link for a list of the codes. Here you go: http://symbolcodes.tlt.psu.edu/accents/codealt.html