One thing I can share is that Annie and Robert fit the profile for a normal couple where the man is older than the woman but not by a large gap. Angela and Michael have a very large age gap and it's not common for a man to want to marry a much older woman, especially one who is poor like Angela. Plus Nigeria is known for scam marriages.
Michael also said on camera that the most important thing is to have a child. Since Angela cannot deliver on that, the chances of them staying together is not good. He also had no job in Nigeria and was supported by Angela which doesn't help either nor does his lack of any special skill (computer, accountant, plumber, electrican) to bring to the US. The officiers generally dont want someone who has a high chance of requiring public assistance and Michael fits that profile.