Thanks for your reply.
I have an idea what causes this issue: You probably created the field group before activating the plugin, which means that the settings for the plugin are not stored in the field group.
This should solve your problem:
Please change on of the settings that are added through the plugin for the repeater and flexible content fields, save the field group. After that, set the settings back to the ones you prefer and save the field group again.
After that, the settings are saved in the field group and the collapse buttons should be visible.
Please let me know if this works for you.