Alerts show in the ATP UI twice a day.
This issue has been fixed in ATP 3.0 release, please upgrade to ATP 3.0 when it releases. If you are not able to wait for the release of ATP 3.0 then open a support case to have a script to be installed to reduce the purging target for events and install a cron job to keep purging track db up to the oldest event.
Note: Tier 1 see Internal notes.