If she's a back sleeper, tummy time is very important and without it she will probably be delayed in rolling, sitting, and crawling. Being upright on your shoulder is not the same thing, because it doesn't develop the muscles in her arms. You might try tummy time up off the floor...on a table, with you right there...on your chest...being held in airplane position...in a playpen...on an exercise ball. Wherever you do it, you should try for tummy every day, at least 30 minutes, although it doesn't have to be consecutive; you can spread that 30 minutes out throughout the day.