When typing the word, right-click it and choose Spelling & Grammar → Show Spelling and Grammar. This will bring up a window similar to the one below. It should say that the word was not found in the spelling dictionary.
Click Learn to teach the word to the current dictionary.
~/Library/Spelling/LocalDictionary only contains some of your corrections. This depends on which language you have currently selected. For example, my
~/Library/Spelling/ folder contains multiple custom dictionaries depending on the language that I have currently set. As well as
LocalDictionary, I have files named
en_GB. Editing these files instead of
LocalDictionary can help set custom words when they refuse to be read from
0 directories, 5 files