Two I-130 485 Petitions filed. 1 accepted, 1 not received

[rodrigo2220]

My parents applied for I-130 and 485.
My dad and mom received an approval letter for I-130.
My mom got her I 485 approval and her green card but my father didn't. Both applications were filed on the same day.
My dad's status update remained on the review of more proof.

My question is if both I-130 got approved why isn't my dad's 485 approval notice has not arrived. What could be some problems with the application.

Thanks

 

[Jackolantern]

They have different life histories and their background checks could take different lengths of time. And both I-485s aren't necessarily processed by the same people at USCIS.