Flight Planning was a pain in the neck for me. I failed twice and the third time had questions on depressurised situations so I didn't have to look for other data and therefore did not run out of time.
Agk is a little tricky too. (2 times for me)
Out of every single exam I've done, those 2 subjects are the only ones I failed.
The actual process of obtaining an answer for fl. plan ?'s isn't hard in itself, it's just time consuming. The chance of pressing the wrong button on the calculator is a factor too. PITA
I too suggest performance and navigation before flight planning, It will help.