Why didn’t somebody catch and flag… what exactly? When they do that, they send an RFE. I’m leaning towards your father got sent an RFE that he didn’t respond to, and that’s why the case was never processed to completion. These are the options (taking into account that you seem unsure as to what application was actually sent):
-IF you were qualified for automatic acquisition of citizenship (which it doesn’t appear you were, but let’s go with if) - an N400 submission would have been denied because you were already a citizen, while an N600 application would have been either approved (in which case you would have your certificate) or an RFE sent if evidence was lacking before it could be approved.
-if you needed your father to apply for naturalization for you (seems to be the case from the dates involved) and he submitted an N400, either you would have had a naturalization ceremony/certificate (didn’t happen) or an RFE would have been sent for evidence lacking to back up the application. (An N600 application would have been denied because you weren’t a citizen yet.)
I am not sure what the timeframe was then, but if you don’t respond to an RFE within the specified time the case is denied as abandoned.
Similarly, if an N400 is approved but you never show up for the naturalization ceremony and don’t reschedule within a specified amount of time, the case is abandoned.
Also, why do multiple FOIAs? Isn’t the only one you actually need your A-file, which will have everything relevant in it?