Need more details about your Father case and we need Sequence of events to understand.
1. Who filed his LC/I-140? Employer A or B
2.Did he work after GC with employer who filed his GC or with A or B or not with A or B but started his own business?
3. Who filed his I-140? Employer A or B? Then at what stage he left A or B? After filing I-485?
4. Date your Father I-140 got approved
5.Date your father filed I-485 and date your I-485 was filed
6.were you on H4 and filed your I-485 with your father I-485?
7Date your father changed the employer after I-485 filing? When did he change?
8. Did your father filed AC21? Does he know about AC21
9. The date your father got GC approved...
10. The date your GC got approved...
11.The date you filed N400
12.Where was your Interview? Let us know the State where your interview was conducted.
13 Why your father did not got US citizenship or filed N400? Does he know he may have problems with his GC?
14. Write the full information that is on N400 denial letter.