Sadly true; spellcheckers aren't complete. They don't contain all the words you will use, just the most common ones. To make matters worse, they don't hesitate to give you a lot of useless suggestions. A writer needs to use other sources to assure a correct selection.
Remember that a spellchecker only checks the spelling of the word you typed. If you typed a homophone, it will never catch that you used the wrong word altogether. Instead of a mispelling, you may have the correct spelling for the wrong word.
Learn from your editor or proof reader. Study the suggestions they provide, use the correct word and try to remember what you have learned.
Thanks Sharon.