Why is it that I was able to do combined science higher in set 5 but not in set 6?

well why did you move down sets? probably the same reason as whatever made you think set 5 wouldnt work presuming it was related to content itself and not something like a different teacher. and if you havent been taught the stuff they are covering in set 6 then this is probably the case. you do not ' just go plain stupid ' there is always something, maybe you cannot focus as well, maybe the change in environement makes you nervous or less at ease, something in your personal circumstances or maybe even just your mindset. try to spend some time analysing whats going on, if it is a test or work in which you did not do well in then ask yourself why it went the way it did.

If you are in a lower set you do feel demotivated but at the same time, at ease and at peace because there's less pressure to achieve top grades. You are basically aiming for a 4. I do think foundation doesn't work for me as a person because every foundation test that I did in school I have failed but higher I passed with at least a 4 (because foundation you need majority of marks to pass).
Set 5 I left due to potential bullying in class and I was bullied by this person online, not because set 5 wasn't working. I was good in combined science and was offered that in set 5 on the higher tier where we all did past papers as a class, set 6 we were all on our own, with us doing foundation papers on our own and some of us expected to do higher when we didn't cover it. It also wasn't combined science, it was separate papers. I actually learnt from my set 5 teacher and was a shame I had to leave.