For words that only have 3 letters, there is no 4th letter. Such as with the word “increase”, there aren't any a lot more letters afterwards and thus should be the word "insert". The word “addict” is in the BIP39 wordlist however, you would have used “addi” for “addict”. Try https://livebookmarking.com/story19588435/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool