Everyone’s brain is different with different capabities for memory. People with incredible accuracy for memory actually have a part of their brain that is larger.
The filing system in the brain is better in some people. They connect a memory in many different centers of the brain and they recall, or pull out the file well, while other people lose or misplace the file. Repeating the same thing over and over again increases the likelihood there will be memory. Children who move at very young ages might forget the places and people before the move, because their entire surroundings change.
Memory also is affected by emotion, and foggy memory or no memory of somethingncan be a defense mechanism. People in depressions can have crappy memories, and I think that helps them in many ways. It can be frustrating if the person still needs to be functioning in daily life though.
Most people I know who have very incomplete memories of childhood remember their childhood as bad ones. I think this can work for or againts the person. There is the possibility that they have blocked very traumatic times. What I also see happen is they block the good times and only focus on bad times, and sometimes overexaggerate having a “bad” childhood. The brain can be very selective. Obviously, serious abuse is serious period, and it doesn’t matter if things were good the majority of the time. I am just talking about more minor things that did not happen often that almost every parent has made mistakes and should probably be given a pass here and there.
When my friends first started having kids and would talk to me about some frustration with them, I almost always could identify with the kid, remembering what it was like when I was young. It seemed like parents either lose the ability or some of them just didn’t remember.
I only have a brief memory of specific instances before age 4. From age 4 on I have a lot of memory. I started kindergarten age 4, which might have something to do with it.