Extreme and/or prolonged stress might be one thing, although struggling to put thoughts into words easily may/may not be part of what can happen with stress (I think it may depend on how young the child is). Of course, if the child/young person is feeling intimidated AND is under stress, I'm guessing that could could create problems with putting thoughts into words. (I was a shy kid, super-verbal, and had times when I'd feel nervous with some intmidating adults and have trouble saying what I wanted to say.)
It's never safe to make guesses about that kind of thing, though, because the person could have any number of conditions that would cause something like that. A lot of kids forget easily, and those who don't really forget often say they do.
The exact same thing is happening to my brother. Me and his teacher were convinced he had ADD It depends on how many hours of sleep you have? It could be Iron deficiency Are you going through puberty being distracted is normal? Have you had any trauma? parents fighting? A break up
Lots of things to consider before jumping to medical conditions. Medical condition need psychiatry and are usaually genetic.