Some organisation’s JIRA instance may have complex or different screen configuration other than the default. Depending on the how the screens are being setup, it would affect how standard and custom fields can be populated when creating or updating issues remotely (i.e. via Web Services).
Take for example the following, where a custom field is associated with three different custom screens which serve different purposes (Create, View and Edit):
In this case, if the “Create Issue Screen” is not associated with the “Test Field”, the custom field will not be taken into account when you perform an Issue creation remotely. This applies to the edit screen as well.
As such, be sure to check your JIRA’s instance screen setting and check if the specific fields that you want to remotely work on is associated with the correct screen.