Custom fields shows as un retrievable on Dynamics Ax 2012 Forms.
Today I was facing the same Issued, I added a few custom fields in AX table. After update data source and dropped custom field at required Place on form. When I run the form, I found that field showed as un-editable and unretrievable written there.
I found that it is cache problem. It solved by click on Refresh element cache. You can found the menu From Tool Menu, Caches and sub menu element Cache,
If problem not solved by this way then restart the Dynamics Ax 2012 service from services.