Robin,
Myles hated tummy time until he outgrew his reflux. About a month later (so about 10 months) he just started hanging out on his belly, pushing up on his arms, rolling front to back and back to front, and finally got to hands and knees, and was crawling in no time. He previously had been way behind, and wasn’t even rolling stomach to back yet. So, he learned all of this in only a matter of weeks. It was amazing.
I was really worried, too and I though he’d never crawl, but as soon as he was feeling better his development just took off. Now’s he’s standing, cruising around furniture, and he can walk with a push toy. I think he’ll be walking in a few weeks.
I’m sure Abby will catch up too.