I kept struggling to get top marks too so I came up with an acronym to help me remember good bits to put in my answers. It's SSEI (pronounced sigh, because that's what you do when you get to the 12 marker!)
S - State the basics of the argument
S - Support the argument with some religious scripture or teachings (you can explain them a bit too, that helps)
E - Evaluate your argument: this is like a mini-conclusion where you link back to the question, it helps you to maintain focus on the statement
I - Consider the impact the argument has on religious believers e.g. some Catholics think abortion is wrong... the impact would be that they would be more likely to suggest giving up an unwanted baby for adoption
I repeat this for 2 arguments for and 2 against, then do my conclusion.
Since I started using this, my answers have improved a lot and I've gotten higher marks; I'd never gotten 12/12 before but I started to get some with this structure (maybe my teacher's just being generous though!) So my advice is to try to remember all the important things you need to include and try to link it back to the question frequently to keep your answer focused.