Two causes are possible. First, your custom dictionary might be empty?you can't edit it until it contains at least one word. Second, if you're running Word across the network, your network administrator may have set custom.dic or its folder as Read Only. Ask permission to create your own local copy of custom.dic, and store it where you can access it.
Custom dictionaries have size limits you need to be aware of. No word can be longer than 64 characters, and no custom dictionary can have more than 5,000 entries or exceed 64KB in size. If you are running up against these limits, create a separate dictionary for the additional words.
You may have hidden grammar mistakes rather than disabled grammar checking. Or you may have disabled Check Grammar as You Type. To completely disable Grammar Checking, choose Tools, Options, and click the Spelling & Grammar tab. Then, remove the checks from the Check Grammar as You Type and the Check Grammar with Spelling check boxes.
When you protect the framework of a form from being edited, Word also prevents users from spell checking the text they enter in that form. If your form contains extensive user text, this can be a significant problem.
To solve the problem, you need a fairly complex macro. Fortunately, the Microsoft Office experts at www.mvps.org have provided one. You can find it (along with detailed directions for using it) at www.mvps.org/word/FAQs/MacrosVBA/SpellcheckProtectDoc.htm.
Sometimes Word doesn't find spelling errors you know it should catch. Assuming that Word's spell checker is turned on, follow these steps:
To ensure that Word spell checks the entire document, make sure you haven't selected specific text when you run spell check. If text is selected when you run spell check, Word will only check the selected text.
Make sure that no text is specifically marked as off-limits to the proofing tools. Select the entire document (Ctrl+A); then choose Tools, Language, Set Language. In the Language dialog box, clear the Do Not Check Spelling or Grammar check box. (Note that Word sometimes enables Do Not Check Spelling or Grammar in documents created by a mail merge, even when you haven't asked for this.)
Run the spell checker again, like this: choose Tools, Options, Spelling and Grammar; click the Recheck Document button; and click Yes. This tells Word to recheck the entire document, not just portions of it that haven't previously been checked.