When you run a broad search that returns thousands of results, you will see that the search indicator will say "MAX". This means that you will not receive any new matches as they arrive.
To ensure you receive new matches, try narrowing down your search criteria. For example, you can search a smaller area, or search back fewer hours. As long as your search returns under 1000 exact matches, you will continue to receive new matches.
For other reasons why alarms/auto refresh may not work please reference this KB Article