unknown positions
I think he means *unfamiliar* positions, where you don't know the plan. I have the same trouble, but usually it's when I'm dumped into a position that is at the end of the database lines somewhere in the middlegame, often a complex position where there are all kinds of threats and dependencies and because I wasn't following the logic as I went though the database, I didn't follow the threats that were in progress before that position was reached.