Focus on the things you're bad at. If you're practising and getting it right at home then theres no reason why you cant do it in an exam.
If youre failing reproducing it at exam level there are typically two reasons:
- you making silly mistakes --> practice doing timed papers and marking as harshly as possible and practise looking through you work and correcting mistakes
- you don't understand the underlying theory behind what you are doing. S1 and chem for instance, its easy to answer typical questions on a subject but if there is any deviation in an exam paper - you cant answer it. --> make sure you understand underlying theory and what the formulas are doing, so read the text book.