Why does my kitten stumble/fall in her harness even though she has pretty good balance?

I'm training my kitten to walk on a harness, she's about 7 months old. When I put her in the harness she stumbles around and sometimes falls over or stays laying down as though it's hard to keep her balance. But when I motivate her with food or especially when I play with her with a toy she has really good balance! She can run and pounds and jump like normal.

Is it just a confidence thing? I've been doing short 5-15 minute training sessions per day to keep from overwhelming her. Is there anything else I can do to encourage her to walk more normally in the harness?

Disclaimer obviously I understand that it's up to her ultimately, I just want to make sure I'm doing everything I can to set her up for success :)