Sleep, I taught dd2 to crawl using the stairs. I would put my knees each side of her to stabilise her lower legs and be right up behind her so that she couldn't topple backwards. And I would alternately move each leg and arm in a crawling pattern up to the next step, supporting her chest/ abdomen with the my other hand to prevent face planting
This was at about 15mos iirc. I would make her do this about 4 or 5 times a day.
IMHO, the stairs provided an 'easier' intro to crawling (which sounds a bit mad, but bear with me) due to the gradient - no huge weight bearing on the arms etc, and easier for kids with low trunk tone. Dd2 was dx with developmental delay at the time and had been having physio and SLT since birth. (she has cerebral palsy)
Dd2 wasn't pulling to stand or anything at this point though.
The physio thought I was completely stark staring bonkers. Or Attila the hun's long lost daughter. But she did learn to crawl. I think the repetitive movement helped not only with muscle memory, but also with making neuro links.
I have, of course, no idea whether it will work for dd, but they do sound very similar.
Make sure you are doing lots of exercises on the gym ball, and on a physio wedge if you have one. Really important to work on her core stability and tone.
Dd2 is hugely bright and has an iq higher than her consultant.
she still has cp. intelligence has v little to do with disability. 
Not crawling can have follow on issues later with some retained reflexes and stuff, but if she just skips crawling and goes straight to walking, it isn't too much of an issue. If she does have low core tone and this is the reason for not crawling, it will be more complex and can have implications for fine motor and self help/ writing etc. so really important to focus on core now.