I get caught out by this more often than I should probably admit. MySQL, out of the box, doesn’t index 3 letter words and below in full-text indexes.
Sadly, a lot of things including brand names and product numbers are three characters.
This is how you fix it and get MySQL to index three-letter words.
1. Add this to your my.cnf
ft_min_word_len = 3
2. Repair any tables that have full-text indexes on them
REPAIR TABLE my_table QUICK;