AS Computing - wandering why my coursework mark is so low

I took Computing as one of my AS subjects last year, hoping to come out at the end of my A-Levels with an A, as its my strongest subject and my favourite!

In my two tests in May/June I got two A's of 80/99 and 84 I think that was out of 99 as well. Then with my coursework unfinished earlier in the term my teacher said it was about a D, but once I'd added things in it would be up to about a B. I was a bit unhappy with this but thought "never mind" - the coursework was tricky as it was quite unclear on how to pick up good chunks of marks. I added and followed everything and spent extra time making sure I'd finished.

I had my mark back, and got a D in my coursework with it marked down 8 marks by the examiner. The rest of my class had poor coursework with every single person marked down between 5 and 13 marks.

Overall I got a B, not too good as AS is where the easiest marks are.

I'm wandering what to do about it, my teachers said that they are annoyed and are trying to contact WJEC about it. However, even in the unlikely event of getting 8 extra marks, the original coursework must have been pretty average anyway, but I dont know how - I think it must be to do with the teaching and outline of the cousework.