When there are 300 or more locations loaded into the app, the pins will not automatically appear on the map.
It's done to keep the page load time as fast as possible. The amount of data to display for more than 300 locations is significant enough to slow down the page, causing the page to become frozen or unresponsive on some browsers and many mobile devices.
On the plus side, by forcing a user to enter their location, you are receiving valuable information. Within the Store Locator app, under Customer Lookup, you have a map containing data for the past six months of customer search locations. You can use this data for strategic planning for future store expansion.