D.K.
Might want to try some "games" to engage the physical......rotate legs as if riding a bike, etc. The more you "prompt" and prime the muscles, the quicker things develop. I had this issues with another infant (I teach) and mom didn't stimulate or play much at all with her daughter. She was 10 months old.
After playing and working with her for 3-4 weeks, she had no problems crawling on her own. Also remember......children learn at different paces.