There are only few reasons why picklist doesn't work, which can be summarized as below:
- Pick map of the new field is not populating the foreign key field of the field.
- Join created to pull the value of the field is not created properly.
- If no values are shown in drop down then pick list is might not compiled yet, or the LOV type and values are not created in administration views.
- If value is not displayed after selecting value from pick applet then wrong field is selected in pick map of the field.
- Update only if null is not configured properly.