Check your junk folder
Spam filters are not perfect so a regular check might find some of those missing emails.
With spam becoming ever more of a problem, filters are getting more and more sophisticated.
However they are never 100% correct and you might find some spam emails slip through in to your Inbox.
Worse still is the possibility that a legitimate email goes to the junk folder. This is why you should check on a regular basis just to make sure you are not missing anything important.
If you do find emails that are going in to your junk folder, the process of correcting the mistake varies between email clients
In Outlook you can right click on the email you wan't to move back to the Inbox and select Junk -> Not Junk
You'll then get a message to say that the email will be moved back to the Inbox.
You will also get the option to tell Outlook that in future, any email from this address is to go in to the Inbox instead of Junk.