Have recently started potty training a still reluctant 3 year old. When she's running around with no knickers on she'll quite happily go for a wee in the potty but if she's wearing knickers she'll just wee in them.
I'm not sure how to make her understand she needs to pull her knickers down, I've read her books and all sorts.
I always kept mine knickerless/pantless when at home at this stage of potty training. They need to get into the habit of always weeing in the potty so they forget they had nappies they coud wee into instead. And of course tons of praise for doing it the potty so she ends up preferring it (tho I'm sure you do this already!). I also let them see me wee into the toilet. Then they wanted to be a big girl/boy and do it like mummy. I think if adults always go behind a closed door, it's harder for little ones to appreciate that potty, then toilet is the norm and the future. Good luck!