Hi CommoMP,
Do be aware there is a limit as to how many words can be added.

As for being unable to add words in capitals, are you sure the same words haven't been input previously? Have you tried editing the custom dictionary manually? I should also point out that adding captialized forms only will result in any lower-case forms being flagged as spelling errors, whereas the reverse isn't true.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]