Implement existing custom fields in customer portal

Hi,

in our test environment based on JIRA 6.1 I try to implement some existing custom fields of my project into the customer portal.

If I a try to use an existing custom field (in create-screen available) it does not work.
If I create a new custom field (of type text or date picker) via administration it's not working.

If I create a new custom field (of type text or date picker) directly via an issue (new feature of 6.1) it works.
If I create a new custom field (of type lable) directly via an issue it's not working.

What's the rule behind this behaviour? Cannot find anything in our weak documentation (really unusual for Atlassian).

Cheers Sascha