I got AAAB at AS.
I started off getting ACCC.
Got some Bs, Cs, and a single D on the way (and mostly As in the subject I initially got A in). One of my so-called 'progress grades' slip sent to my house was CDEE, even though I always exceeded those grades in every single assignment, never got an E grade in any of my subjects, and usually got solid Cs, with the exceptions of the one subject I was very good at, in which I got As, a B, and a C and one subject in which the teacher gave me a D.
My mock grades were ACDE, with D in the subject in which I got the B and E in one of the subjects in which I got an A.
I studied for my mocks more than I studied for the real exams, so I am guessing my teachers were just very harsh markers or did not like me much (one of them outright hated me and tried to get me removed off the course).
If you revised so much and only got B-C, I would doubt the quality and accuracy of the marking, to be honest. Not saying all such grades after much revision are undeserved, just that in my case, they were, so I would not be surprised if someone was in a similar situation.
As for how I did it, I read my notes over and over again. Most of my studying was done by using a timetable in which I studied 6 hours per subject every week (yes, that was 24 hours of revision a week in total). I only managed to keep it up for three weeks, but it was enough to get most of the information in my head. I recommend you keep such a schedule all year because I am just fortunate to have been blessed with an amazing memory and thus did not need an extremely huge amount of time to memorise everything. I did past papers, read mark schemes until I understood them, read examiner's reports, and read model answers (read a lot of the good ones, a few terrible ones, and like one or two average ones).
And most importantly, do not panic! A-levels are not that hard in content. The hardest parts of A-levels are coping with the pressure and not having a mental breakdown.
Remember: your mock grades are not your real grades and definitely do not represent your maximum potential (unless you get straight As, in which case there is no higher grade to get). None of the grades you get all year have any bearing in the exam. You simply have to study and do well in the exams and all will be fine.