1. Ensure that you shared using the correct email address.
2. If you shared using the 'Email Shared Link' option in Box (shown below), the recipient's spam settings may have affected the delivery of share notifications. Try copy/pasting the link from the Share pop-up and sending it directly using your email account.
To prevent notifications landing in spam, you and those you are sharing with should 'whitelist' the following email address: noreply@box.com
For more specifics on this topic from Box click here.
Find out how to 'whitelist' for common email clients here.