Some days, there really isn't a lot of creative fluids left. For me, this is if I'm tired, stressed or something RL is just completely messing with my head. Sometimes, you just need a day off.
Most of the time, for me at least, it comes down to anxiety. It's not so much that the creativity isn't there, as much as it's being completely drained all manner of fears. It sounds like this might be what you're facing, right? A bit similar to the fear of the blank page that some authors feel.
I want you to think about what goes into making an amazing OC, and what's fair to expect of yourself. Is it fair to expect you to be able to come up with one in five seconds? No, right? Five minutes? Five hours? If you're doing it for fun, why is there a timer on your performance in the first place? Does it have to be amazing, or is it fine to make something that is just okay? Okay and fun?
Anxieties can convince you something is deeply wrong if the ideas don't start flowing from the first moment. And then it's even harder to make them flow, because you're anxious. It'll make you worry about every perceived mistake, instead of allowing you to shrug your shoulders and just send something imperfect into the world. Barring the fact that just about everything is imperfect. Like, I don't do fandoms, but I've only a sliver of a doubt in my mind that most people writing fandoms don't nail the characterizations perfectly. I wouldn't be surprised if a lot just don't care; they want to see their beloved canons fuck.
For me, getting better with my anxiety has really been about pushing into it and letting myself experience it (In limited doses). It's been about reminding myself of various truths from time to time, such as "It's okay to make mistakes" or "I can't expect myself to do this in no time, I need to give myself time to work this through" or "I can always edit it later, it doesn't have to be perfect right away. In fact, it doesn't have to be perfect at all". Resting on your feelings, focusing on them or meditating on them may help as well. Usually, anxious feelings needs to be processed rather than ignored.
Best of luck!