Because of the Strava API limits, Stravanity can only load 300 segments every 15 minutes, with up to 3,000 segments per day. These limits are shared by all users.
The 15-minute limit is reset at natural 15-minute intervals corresponding to 0, 15, 30 and 45 minutes after the hour, and the daily limit resets at midnight UTC.
If you reach one of these limits, the only thing you can do is to wait and come back later to load new segments.