Help, PLEASE!!!

I need some advice. Avery was such and easy kid. No blankie, No bink-bonk (pacifier), slept great, weaned her from the bottle very easily, and potty trained very easily. All around an easy kid. Fast forward to Adison. She is very easy also, but had a bink-bonk until a month ago, still has a blankie, and is having a hard time potty training. I am a firm believer that they will do it when they are ready. It is just too frustrating when you make them. Well, Adi is ready. She gets so excited to wear big girl underwear. Here is the problem, she will not go on a little potty, so I got her a toilet seat that goes over the regular toilet. She will finally sit on it, but refuses to go. I think the problem is that a few monthes ago she fell in and scratched her back. Well, she insists on wearing "Big Girl" underwear everyday. She just "HOLDS" it all day long. She has been in underwear since 8:30 a.m. and is still dry. Has not had an accident or gone on the toilet, and it is almost 4:00 p.m. I know she has to go, so I will be putting a diaper on her soon. Any suggestions on how to get her to GO????!!!!