I think with toilet training there is a difference between showing an interest and actually being ready. Like you with my first I put her straight in underwear and she was trained in a week and dry at night after 4 days. She just got it and I think that is exactly the thing, she understood it and was able to control it. I think that perhaps 20 months was to early for her to get it even if she was interested in it, that's just my impression from the info I have. I personally would either stop toilet training altogether and go back to it in 4-6 weeks when she is a little more physically able to control her bladder, or get rid of the pull ups altogether, put her in underwear and opt for conditioning her initially with lots of reinforcement during this time about what she should say when she needs to go, and that we don't pee in our pants etc. Neither one is necessarily ideal, but what you are doing right now isn't working either so they would be my suggestions for next course of action.