With ! not working with words in the user dictionary, is this with Chinese or English? My best guess here is that by adding the word to a user dictionary you've caused it to return a more exact result for your search than it would have otherwise and that's somehow displacing the flashcard-matched one.
With * not working, that's my mistake, I forgot that user dictionary headword wildcard search is limited to the first 4 characters for performance reasons - not really optimized around searching for sentences.