Adding or removing an event in Altru can throw off the vanity URLs for the rest of the events. Use the following steps to correct the issue.
Under Events the require Registration (Vanity URLs) there is this section:
Altru Events 

If these names are not unique follow this procedure:
•    Registered Events of the same name must have their web forms published in chronological order.
•    If an event is removed, all events of the same name and date must be republished in chronological order.
•    If an event is added, all events included in the one added with the same name and date, must be republished in chronological order.

The solution is to (in Altru) un-publish and republish the events in question, then re-run the sync in Wordpress. 
Manually Refreshing the Calendar
  • There is a refresh button on the import feed configuration section of the Event menu in WordPress polls the proxy service for updates. 
  • A Proxy is an intermediary service between Altru and Wordpress. 
  • There is no way to ask the proxy to refresh so if the proxy hasn’t updated, the manual refresh will not do anything.
 If refreshing the feed does not work it runs automatically every hour and will show up if you wait.

Also see: WordPress calendar does not show events from Altru