In order to use the follow up feature, you must enabled it for each custom activity-type in Activity Settings. To enable, navigate to Settings | Activities and then scroll towards the bottom to find the Follow-Up option and checkbox icon (). Check the box to enable follow-ups on the selected activity-type.
You may also set what activity type the follow up activities should use. This is simply a default. Selecting this option does not preclude a user from changing the default when on the activity form later.