The further along you are the more you'll be able to feel! If they were unable to feel it's probably because your uterus is still below the pelvic bone!
That's what they're feeling for...to see where your uterus starts and ends...the further along you are, the bigger your uterus will get as well. They usually use a measuring tape to measure your stomach aka uterus from bottom to top. The centimeters should match up with the weeks of gestation/pregnancy. I.e. If you're 32 weeks, your belly from bottom to top will measure 32cm.