You're sustaining gift frequency option may not display if you have an enforced end date for your duration period.
  1. Select Fundraising > Donation Management
  2. Select the All Donation Forms tab
  3. Click Edit in the actions column of the donation form
  4. Click Design Donor Screens
  5. Click Edit next to Donation Form
  6. Select the Flexible Sustainer Donation Level data element
  7. Click Save Order and Edit Selected
  8. For Duration Period, select the radio button for Selected Duration Only
  9. Click Save