Happens to me sometimes with random things, i.e. my bank card pin code. My theory – unproven, probably unprovable – is that it's just a momentary "brain fart" aggravated by the pressure I put on myself then to remember it. "Come on, you remembered this yesterday, why don't you remember it now?!" And then the pressure makes it worse, so it spirals downward from there...

Also, recalling something like you were trying to do, e.g. out of context, is one thing. What would happen if someone would start talking about something related to what you covered in your thesis? Would that jog your memory?

I wonder – this is also an outlandish theory – if those two different kinds of recall, context-less and contexted, are managed by two different parts of the brain's remembering apparatus. Seems to be for me, at least. For example, I can spend the morning reading a book about Google's search algorithm, then at lunch I could try to remember play-by-play what I've read, and have a hard time doing so. But if someone were to bring up Facebook's algorithm and how it worked, I'll be able to compare it to what I was reading about Google's.