Custom fields are created as types and the drop-down type is designed to enable you to only be able to select one option at a time.
1. If you can only select one option at a time then you have probably created your custom field(s) as Drop-Down type.
2. If you want to be able to select more than 1 option at a time then you need to create your custom field as Multi-Select.
3. You can edit your existing custom field (or create new) from drop-down to multi-select and then re-edit your contacts/organisations to select your desired options.
That should sort it but if you have any further questions/problems then please contact our InTouch Support Team.