Hi, ATPL FPL has always been the hardest exam - partly (it seems) tradition, and partly due to lack of clarity of what is expected.
This is not an exam you can pass by doing practice exams until you know all the answers - you have to actually understand the question, the situation described, how to do a flight plan, and (the hard bit) what SOPs are to be used.
You are NOT expected to produce the most accurate FPL; nor the quickest (though you can't be slow). You are expected to follow a specific set of (poorly defined) rules.
That last bit is the key - the practice exams and notes will test that you can follow a process correctly; not that you really understand the situation and the SOPs. I found out the hard way that many of the notes and practice exams only test part of what you need to know (the process); they don't test you're understanding of the situation described (how to use alternates); and sometimes their assumptions on winds/temps/weights don't match CASAs SOPs. Look carefully through the CASA exam guide - you're looking for how to work out exactly what winds, weights, temps etc you are to use.
No point doing multiple attempts in one week! You obviously were under-prepared in some way - take your time and prepare yourself well.