As Ellen said, the delts are lots of tiny little muscles. Because they burn out quickly, I never do more than 10 reps. For Shoulders, I'm interested in size, since the larger my shoulders look, they smaller my non-existent waist will appear.
It takes a looooong time to develope shoulder strength. In 5 years, I've managed to move from 5 lb dumbbells to 20 lb. for lateral raises. I just changed my form to try to change the shape of my mid delts, and had to move back to 17.5 for my heavy set. It really takes a long time to be able to move up in weight.
One of the seemingly small influences I had at my gym was convincing the owner to buy weights in one pound increments from 5 to 12 pounds. He didn't believe me at first that one pound made any difference until I started working with two clients who were coming back from rotator cuff surgery. But now ALL the women are thanking me!
If you don't have smaller increment weights, use drop sets to help yourself progress faster. If you can only do 6 reps at 10 pounds, FINE! do the 6 reps, and have 8 pounders lined up and grab them to finish out the set. You'll get stronger faster than waiting until you can complete full sets before you move on.
Mel