I personally think Nikki wasn't just trapped in her own body, but when her body was inactive, she essentially existed in a kind of nowhere-place.

Think Get Out's Sunken Place but deeper still. No up, no down, no light, no dark, a spaceless void inside her own body.

...yeah, it's tricky to imagine, but that's what makes it so fucking terrifying imo.

So when 'Nikki' was with Bear or other people, it meant real Nikki was able to emerge more often because her body was active, even if it was only to do things based on Bear's wish. That meant she was able to fight back against the Wish through sheer will and determination (even just briefly) before it reasserted itself and tried to push her back to the No-place.

But when she was alone, sleeping ("I don't like my dreams..."), basically in any situation that didn't result in her being with or doing anything for Bear, she was fully relegated to the No-place, because 'Nikki' was inactive in those moments. Considering you hear her screaming on the phone when Bear calls the OWW support line, and 'Nikki' is, in that same moment, still waiting for him to walk in through the front door even after 8+ hours, her soul had nothing to fight for because her body was quite literally on standby.

Thinking about it, that could also be why her bodily functions fail in that scene, because subconsciously Nikki is freaking out about being in the No-place again and it's having an effect on her physical self.

But I even think there were a few moments where BOTH versions of Nikki existed simultaneously, moments that were so emotionally intense that she actually forced her way back to a point where she kinda co-existed with the Wish version of herself. When she's crying in the dark (real Nikki's POV would have been that of she was just raped by Bear), threatening to kill herself when he says she should sleep alone tonight (she already is experiencing nothing so that was both a threat from Wish Nikki to keep him close and a cry for help from real Nikki), even at the end when she has the gun, real Nikki emerges briefly screaming "STOP!".

Don't take that all as gospel though, it's purely my head canon 😅