R.R.
I wouldn't say she's unable. My guy was a late walker, and very cautious. i talked to his pediatrician at his one year check up and she said the same thing, if he's not walking by 18 months we'll look into it then, and not to try and force him to walk. And he was walking before 18 months so there was no problem.
He also dragged one of his feet when he crawled, I looked it up online and found it was completely normal, many children do it, I think it was called "scooting." Babies typically begin crawling between 6 and 10 months so your daughter is right where she should be. There's many forms of crawling and they're all normal and fine.
I also read this ~ "The important part of this stage in development isn't the specific method of locomotion, according to DrGreene.com, a website created by San Fransisco pediatrician Alan Greene, but the fact that the infant is becoming mobile at all. Any movement that provides forward momentum is worthwhile at this stage, and parents should encourage babies to explore their surroundings by placing toys just out of reach and setting up obstacle courses made of pillows, cushions and boxes." So just keep encouraging her to move and when she's ready she'll walk, which is what our doctor told us.