From my UMAT experience (disclaimer: I've never sat UCAT), it's definitely possible to burnout with practice. It's possible you're not engaging as fully with your practice sessions as you have previously, or have become a bit complacent and are making little mistakes (this is something I definitely noticed I would do). It's also possible you're reaching your max, in which case, improvements won't be anywhere near as steep as they might have been at the beginning, and it can be easy to become disheartened, so it's important to keep things in perspective, take a break, do something else, and then come back to it.
Another thing I did when addressing UMAT (I was needing to improve an already decent 92nd%ile to something much higher for UTAS non-standard entry) was to honestly evaluate my practices and figure out my weaknesses. There were some S1 questions that I knew I could spend a disproportionate amount of time on and still not be confident to have the right answer, and so I'd have to rationalise the time I was taking and instead make the decision to 'educated guess' and move on to the next question. Recognising your own strengths and weaknesses, doing a bit of an analysis of the tasks and how suited they are to your skill set, is invaluable, imo. I ended up getting 98%ile the second time round, and put this down to experience (the best experience for UMAT was UMAT, hand's down! I suspect it will be the same for UCAT!), and being smarter about my prep.
