JIRA Service Desk does not allow every type of custom field to be displayed on the customer portal. This is not a restriction for add-ons, but also a restriction for build-in custom fields of JIRA itself. Simple fields like, Group Picker, Labels, Project Picker etc. are not compatible with JIRA SD. There is a popular improvement request, JSD-86, you can vote up if you want to Atlassian to allow more custom field types on customer portal of service desk. Note that although a field is not compatible with customer portal, it is still useable in other locations inside a JIRA Service Desk installation.