Same here. I absolutely crushed GCSE maths, cruising to an A without any difficulty (on one of my mock papers I only got 99% due to a silly labelling error on a graph that lost me one mark and I'm still upset about it today 32 years later!) but A-Level maths completely flummoxed me.
Actually it was Pure maths that was my nemesis, while Stats I was still quite good at - while they didn't give you a break down of your grade, based on my mock exams, my "E" overall was probably a "U" in Pure and a "B" in Stats. What I should have done was after the mock, go to my teachers and tell them I was going to drop Pure and only do Stats at AS-level instead (which was an option), but I didn't have the guts back then and tried to stick it out.
I had the same problem
I was so good at maths at "O" level that the maths department were trying to persuade me to do the Maths and Further Maths "A" levels
but when I started just doing the normal "A" level meths the pure maths just confused me
I think it was partly the teacher who would solve the problem on the board
then when I went and told him that I couldn;t do it last night for homework he just pointed to the solution
He didn;t seem to be able to understand that I knew everything he said
I just had problems working out how to start in the right direction
anyway - I ended up with a D
which also meant I had to do an extra maths class in the Chemistry department at University
who had a teacher who was even worse
in fact one day he was going through some "A" level type maths with us "remedial maths class" people and none of us had a clue what he was talking about
so after we went to T2 coffeee bar (where we pretty much lived when not in Chemistry labs) and were talking about it
I ended up saying lets just ignore our notes and look at the actual problem he was solving
we could ALL do it - he was actually explaining something we already knew and understood
and we still could not understand what he was saying
all of which helped me a lot when I became a teacher many years later
I knew to always try to find out WHY a pupil doesn;t understand
and not just explain it again