I am preparing for my Navigation exam but I keeping having mental blocks, especially with the whizzy wheel.
You're not the only one.
I've done my X-Country and currently just short of 37hrs. I can plan any route the normal way i.e. knowing the wind etc but like you my mind stutters when I am asked about other elements. The worst thing for me is that I'll revise for 3 evenings one after the other with a goal of sitting the exam on the 4th day (usually a weekend) but due to the weather the school has been closed causing a delay, by the time I get a clear day my confidence is low and I need to go back to the drawing board.
I'd like to get the Nav exam out of the way soon, I'll probably have to book a day off work next week if I can't sit it this weekend.