Try the following steps until the issue is resolved: 
  1. Note the price types used on the checked in group sale reservation.
  2. Click on the itinerary of the group and note the scheduled program event name.
  3. Go to Tickets > Program Search and open that program.
  4. Go to the Event List tab and click on the event that was used on the group sale.
  5. Go to the Prices tab.
  6. If the price types on the group sale are not listed here, add them back.
  7. Then, change the number of visitors on the group sale.