Have a think about what the problem is... what's not happening in the exams you're doing now that was happening when you were doing gcse and getting results you were happy with.
Think about why are you answering exam style questions when they're in the book but failing when the questions are in an exam.
The differences I noticed when going up to A level (a long time ago) were.
1. pacing - new material comes along at a quicker rate
2. difficulty - you often had to think about it a bit harder
So if you're trying to put the same amount of effort in as you were at gcse that might not be enough to get results you're happy with anymore... basically you know you've put enough effort in when you feel secure in your knowledge and are pretty confident that the examiner can't catch you out on that subject.
Answering the exam questions is IMO a two part process
first you've got to 'unpack' the question - find out what it's really asking you about and identify the relevant physical theory... e.g. if the question describes winching a massive object out of a mineshaft with an electric motor you should start thinking about energy conversion between electrical energy and gravitational potential energy.
second part getting suitable data and formulas, rearranging them and plugging in the numbers.
Obviously there are exam techniques to use - e.g. don't waste time on a question you are stuck with, move on to one you can do and go back to the hard question at the end. Spend more time on high mark questions than low mark questions etc.