How do you write an A standard AS level history piece?

Depends on the board you're on.

I'm doing A level now, but on our AS board we had to do a passage based q with four sources and a "using these sources and your own knowledge..." one, followed by an essay. The real key to getting a high grade in history at A level, I've found, is technique. You're probably getting a B/C because although you "know most of the stuff on the course" you're just being descriptive rather than analytical.

If you have the passage based q you need to group the sources - which ones agree with the statement in the q and which disagree - and then cross-reference them as you go through. First argue to agree with the statement, citing the sources that are in accordance and explain the different ways they support the statement. Then do the other side. It's really rather simple and you don't actually need as much factual knowledge as everyone assumes you do for history.

My advice would be to ask your history teacher for a technique advice sheet explaining how best to structure your answer. If they don't have one, ask them to explain the structure to you, because I bet that's your problem.