Hi. I think your 21mo old is acting like a normal 21mo old. She is having too much fun being naked and probably gets excited and forgets. I would have her help you clean up the pee on the floor and just ask her "where do you do pee?" and let her answer. That way you know you've reminded her, she gets and idea of the consequences and she thinks about her actions too. I wouldn't expect too much from her she's still little. Good luck!
ps, I've trained a boy and a girl.

When I potty trained my daughter I took her to the store and let her pick out some pretty panties. When she went potty on the potty she got to wear them and if she had an accident I told her that we had to put on the ugly training pants. Although, since your daughter is naked that might not work for her. I do know someone that put the potty in front of the t.v. and would put her child on it ever so often. She sat there with her daughter and gave her M & M's until she went potty. You might try something like that. Good Luck to you