If you have added a location, and it's not appearing on your Interface (e.g. on the map or in the listing) there may be a few reasons why:
Has the location been Geocoded, (assigned a latitude and longitude) as indicated by a green checkmark in the geocoded column?
Are your Interface settings limiting the number of results displayed under Data Settings? If you can search for the location and find it, but it's not displayed by default, increase the total number of records displayed by default to ensure all locations are displayed when the map loads. Note that you should not exceed ~1000 locations to ensure good performance on slower computers.
Is the location published as indicated by a green checkmark in the Published column?
Is the location appropriately categorized to correspond with your Interface settings?
Do your Interface settings include a country selection? Does the missing location have that same value in it's country field?
Does searching for a nearby postal code within a large radius return the location? Was it properly geocoded?
Does keyword searching return the location? Was it properly geocoded?
Does the Interface include a Category selection? Are you sure the location was categorized with that Category?
Is the location very close to another location, and hidden behind that location when zoomed out? Does zooming all the way in reveal the location?