Certain characters in search cause a fail & and : are 2 I am aware of. I’ve not had a problem with , and '.
I also get failed searches on incomplete words like a word missing it’s beginning letter like elery instead of celery. I don’t know if capitalization is a problem or middle of the or end of the word missing characters is also a problem.
I reported this problem through an email address I had because the form was preventing me from submitting the problem. If someone gets this in support all the better.
Some characters are excluded from searches because they have special functions as search parameters. & and : are some of them.
Capitals are not taken into account, so searching for celery will return occurrences of Celery. Partial words are also returned so elery will return celery.
Two things you can check if this is not working:
Make sure you have selected the correct fields and folders for your search. That is done using the down arrow in the search box. If you only have subject selected, that is not going to search for words in the message body.
Make sure you are using the latest version available for download from the Release History. The current version has no issues and correctly returns celery when elery is used for a search.